Basic Economic Ideas
A-Level Economics Topic 1 18:39 English narration · English + 中文 subtitles burned in
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Write down everything you want.
把你想要的东西全都写下来。
A phone, a bigger home, travel, a good education.
手机、更大的房子、旅行、好的教育。
The list never ends.
这个清单永远写不完。
Now look at what the world has: a fixed amount of land, of workers, of machines, of time.
再看看这个世界拥有的：有限的土地、有限的劳动力、有限的机器、有限的时间。
<slow>Wants are unlimited.
欲望是无限的，资源却不是。
Resources are not.</slow> That one gap is where economics begins.
这一道缺口，就是经济学的起点。
This is topic one: the ideas the whole course is built on.
这是第一个专题：整门课程赖以建立的基本观念。
Scarcity, choice and cost.
稀缺、选择与成本。
What resources are, and how countries share them out.
资源是什么，以及各国如何分配资源。
Let's begin.
让我们开始吧。
Six stops on the road.
路上有六个站点。
First, scarcity and opportunity cost — why every choice has a price.
第一，稀缺与机会成本——为什么每个选择都有代价。
Second, how economists think: models, facts and opinions.
第二，经济学家如何思考：模型、事实与观点。
Third, the four factors of production.
第三，四种生产要素。
Fourth, the systems that share resources out, and the price that guides them.
第四，分配资源的体制，以及引导资源的价格。
Fifth, the production possibility curve.
第五，生产可能性曲线。
Last, types of goods — free, private, public, merit and demerit.
最后，物品的类型——免费、私人、公共、优值与劣值。
Keep that map; each stop earns marks.
记住这张地图；每一站都有分数。
Resources are the things we use to make goods and services.
资源是我们用来制造物品与服务的东西。
Goods are things you can touch — food, phones, clothes.
物品是你能摸到的——食物、手机、衣服。
Services are work done for you — a haircut, a lesson, a bus ride.
服务是别人为你做的工作——理发、上课、坐车。
Both come from scarce resources, so both cost something.
两者都来自稀缺资源，所以都有代价。
Human wants for them never run out.
人们对它们的欲望永远不会耗尽。
That is why every society faces the same economic problem: how to use limited resources when wants keep growing.
这就是为什么每个社会都面对同一个经济问题： 欲望不断增长时，如何使用有限的资源。
Resources are finite. Wants are unlimited.
资源是有限的，欲望是无限的。
That gap is scarcity — the fundamental economic problem — and it forces every society to answer three questions.
我们把这道缺口称为稀缺性， 它迫使每一个社会都要回答三个问题。
First, what to produce — hospitals, or shopping malls?
第一，生产什么——是医院还是商场？
Second, how — with workers, or with machines?
第二，如何生产——靠工人，还是靠机器？
Third, for whom?
第三，为谁生产？
Who actually gets the output?
产品做出来以后归谁？
Every system you will study answers those three differently.
你将学到的每一种体制，对这三个问题的回答都不相同。
Because resources are scarce you must choose, and every choice costs you what you did not choose.
因为资源稀缺，你必须选择，而每个选择都要以你没有选的那个为代价。
Opportunity cost is the next best alternative you give up.
机会成本是指你放弃的次优选择。
Notice those words — next best. Not everything, just the best one.
注意「次优」这两个字：不是你放弃的一切， 而只是其中最好的那一个。
A consumer buys a phone and gives up the holiday.
消费者买了手机，就放弃了那次旅行。
A firm uses its factory for cars and gives up the trucks.
企业用工厂造汽车，就放弃了卡车。
A government spends on hospitals and gives up the schools.
政府把钱花在医院上，就放弃了学校。
Say the definition every time you write it: opportunity cost is the next best alternative given up.
每次写定义都要说全：机会成本是所放弃的次优选择。
Consumers face it when they spend.
消费者花钱时面对它。
Producers and firms face it when they choose what to make.
生产者和企业决定造什么时面对它。
Governments face it when they spend tax money.
政府花税收时面对它。
The exam marks the words next best.
考试给分看的是「次优」这几个字。
If you only say "what you gave up", you leave a mark on the table.
如果你只说「你放弃了什么」，就丢掉一分。
Economics is a social science.
经济学是一门社会科学。
It studies how people choose, builds simple models, and tests them against data.
它研究人们如何做选择，建立简单的模型，并用数据检验。
But the real world moves everything at once.
但现实世界里所有因素同时在变。
So economists hold everything else still and change one thing at a time.
于是经济学家把其他条件都固定住，一次只改变一个因素。
That has a Latin name: ceteris paribus — all other things staying equal.
这有一个拉丁文名字：ceteris paribus——其他条件不变。
Microeconomics zooms in on one market, one firm, one consumer.
微观经济学聚焦于一个市场、一家企业、一位消费者。
Macroeconomics pulls back to the whole economy.
宏观经济学则拉远来看整个经济。
Here is the classic example.
经典例子在这里。
If the price of coffee rises, people buy less coffee, ceteris paribus.
如果咖啡价格上涨，人们就少买咖啡——在其他条件不变时。
We hold incomes, tastes and other prices still, so only the coffee price changes.
我们把收入、偏好和其他价格都固定住，只让咖啡价格变。
Without that assumption the story falls apart — maybe incomes rose at the same time.
没有这个假设，故事就站不住—— 也许收入同时也涨了。
Macroeconomics then studies total output, jobs and rising prices across the whole economy, not one coffee stall.
宏观经济学接着研究整个经济的总产出、就业与物价上涨， 而不是一家咖啡摊。
Now a distinction the exam tests every year.
接下来是一个考试每年都考的区分。
A positive statement is about fact — it can be checked against evidence and found true or false: a higher petrol price reduces the amount people buy.
实证表述是关于事实的—— 可以用证据检验真假：「油价上涨会使人们的购买量减少」。
A normative statement is about opinion: the government should make petrol cheaper.
规范表述是关于观点的：「政府应当让油价更便宜」。 这句话建立在价值判断之上。
That rests on a value judgement. Here is the test. Hunt for the word should, or ought, or unfair.
判断方法在这里：去找「应当」「应该」「不公平」这类词。
Argue with it, but you can never settle it with data.
你可以和它争论，却永远无法用数据定论。
A value judgement is a personal view about what is good or fair.
价值判断是关于什么是好、什么是公平的个人看法。
Two people can agree on every fact and still disagree about what should be done.
两个人可以同意所有事实， 却仍对应当做什么意见不一。
One may say petrol should be cheaper to help drivers.
一个人可能说油价应当更便宜来帮助司机。
Another may say it should stay high to cut pollution.
另一个人可能说油价应当保持高位以减少污染。
Data settles the positive claim about how much people buy.
数据可以定论人们买多少这一实证主张。
Data never settles the should.
数据永远无法定论那个「应当」。
Resources come in four kinds, and each earns its own reward.
资源分为四类，每一类都赚取自己的报酬。
Land is everything nature provides — fields, water, oil, minerals. Its reward is rent.
土地是自然界提供的一切—— 田地、水、石油、矿产，报酬是地租。
Labour is human work, of body and mind. Its reward is wages.
劳动是人的体力和脑力劳动，报酬是工资。
Capital is anything made in order to make other things: machines, tools, factories. Its reward is interest.
资本是为了生产其他东西而制造出来的东西：机器、工具、工厂，报酬是利息。
Enterprise is the skill of combining the other three and carrying the risk.
企业家才能是把前三者组织起来并承担风险的能力，报酬是利润。
Its reward is profit. Learn the pairs.
要把这些配对记熟。
Look at this open-pit mine.
看看这座露天矿。
Land is not only green fields.
土地不只是绿色的田野。
It includes oil, coal and minerals that firms dig out.
它还包括企业挖出的石油、煤炭和矿产。
Some of those are non-renewable resources — once they are gone, they are gone.
其中有些是不可再生资源——用完就没有了。
Enterprise is supplied by the entrepreneur: the person with the business idea who risks their own money.
企业家才能由企业家提供： 那位有商业想法、并拿自己的钱冒险的人。
Without that risk-taker, land, labour and capital stay idle.
没有这个冒险者， 土地、劳动和资本就会闲置。
Drill the four pairs until they stick.
把这四组配对练到烂熟。
Land earns rent.
土地得地租。
Labour earns wages.
劳动得工资。
Capital earns interest.
资本得利息。
Enterprise earns profit.
企业家才能得利润。
Mix them up on a multiple choice and the mark is gone.
选择题搞混一组，分数就没了。
Capital means man-made tools of production — not money in a bank account.
资本指人造的生产工具——不是银行账户里的钱。
Money buys capital; money itself is not a factor of production in this course.
钱可以买资本；在本课程里，钱本身不是生产要素。
Specialisation means doing only what you are best at, and trading for the rest.
专业化是指只做自己最擅长的事，其余的靠交换获得。
Inside a workplace it becomes the division of labour: a big job is cut into small tasks, each worker repeating one.
在工作场所内， 它就成为劳动分工：一项大工作被拆成小任务，每个工人重复其中一项。
The gains are real. Workers get faster, no time is lost switching jobs, and output per worker rises, so cost per unit falls.
好处是实实在在的：工人变得更快，不必在不同工作之间切换而浪费时间， 人均产出上升，单位成本下降。
But there is a price.
但也要付代价。
The work is boring, so people leave.
工作枯燥，人们会离职。
One absence stops the line.
一个人缺勤就会让生产线停下来。
And one narrow skill is dangerous.
而只掌握一项狭窄技能，是很危险的。
A car assembly line shows both ideas at once.
汽车装配线同时展示了两个概念。
Machines and robots are capital.
机器和机器人是资本。
Each worker repeats one task — that is the division of labour.
每个工人重复一项任务—— 那就是劳动分工。
Specialisation also works at firm and country level.
专业化也在企业和国别层面运作。
A firm may only make engines.
一家企业可能只造发动机。
A country may only export coffee and import machines.
一个国家可能只出口咖啡、进口机器。
Trade then lets everyone enjoy more than they could make alone.
贸易让每个人享用的比独自生产的更多。
Without trade, specialisation would leave you with only one good.
没有贸易，专业化会让你只剩一种物品。
Resources must also move to where they are needed.
资源还必须流动到需要它们的地方。
Occupational mobility is how easily a factor moves to a different use — a teacher who retrains as a nurse.
职业流动性是指一种要素转到另一种用途的难易程度—— 比如教师改行做护士。
Geographical mobility is how easily it moves to a different place; land scores zero, since a field cannot be carried.
地理流动性是指它转到另一个地方的难易程度； 土地在这一点上得分为零，因为一块田是搬不走的。
Labour is often stuck too: family ties, housing costs and missing skills.
劳动往往也难以流动： 家庭牵绊、住房成本，以及缺乏技能。
So who answers the three questions?
那么由谁来回答那三个问题呢？
That depends on the system — and comparing the systems is a standard essay.
这取决于经济体制。
In a market economy, firms and households own the resources and the price mechanism decides.
在市场经济中， 企业和家庭拥有资源，由价格来决定。
You get choice, low prices and a reason to invent — but a wide gap between rich and poor, and some goods nobody supplies.
你会得到选择、低价和创新的动力—— 但也会有巨大的贫富差距，还有一些物品没人供给。
In a planned economy the government owns and decides: a smaller gap and basics for all, but weak incentives and shortages.
在计划经济中，政府拥有资源并做出决定：差距较小，人人有基本保障， 但激励不足，短缺常见。
In a mixed economy both sectors act — and every real country is mixed.
在混合经济中，两个部门都发挥作用—— 现实中每个国家都是混合的。
A planned economy is also called a command economy — the state commands what is made, how, and for whom.
计划经济也叫指令经济——国家下令生产什么、如何生产、为谁生产。
In a mixed economy the private sector — firms and households — and the public sector — the government — both make decisions.
在混合经济中，私营部门——企业和家庭——与公共部门——政府——都做决策。
Planned systems often create shortages and surpluses and change slowly.
计划经济常造成短缺与过剩，而且变化缓慢。
Mixed systems try to keep the good points of both, but the right balance is hard to find.
混合经济试图保留两者的优点， 但正确的平衡很难找到。
Real systems sit on a spectrum.
现实体制落在一条光谱上。
A pure free market is at one end.
一端是纯粹的自由市场。
Full government planning is at the other.
另一端是完全的政府计划。
Almost every country sits somewhere in between.
几乎每个国家都坐在中间某处。
China's economy, for instance, is officially a socialist market economy: markets set most prices, while the government guides key sectors and owns major enterprises.
例如中国的经济官方称为社会主义市场经济： 市场决定大部分价格，同时政府引导关键部门并拥有大型企业。
Do not pretend any real country is a pure extreme.
不要假装任何真实国家是纯粹的极端。
In a market nobody is in charge — and yet resources still find their way.
在市场中没有人发号施令——可是资源仍然找到了去处。
The price does three jobs at once, and the exam wants all three by name.
价格同时完成三项工作， 考试要求你把三项都点出名字。
First, signalling: a rising price tells everyone this good is wanted here.
第一，信号传递：价格上涨告诉所有人， 这里需要这种物品。
Second, incentive: that higher price rewards producers, so they supply more.
第二，激励：更高的价格奖励生产者，于是他们供给更多。
Third, rationing: a high price shares a scarce good among those able to pay.
第三，配给：高价把稀缺的物品分配给有能力支付的人。
<slow>Three jobs, one number.</slow>
三项工作，一个数字。
Picture the price in the middle.
把价格画在中间。
Signalling shows where resources are wanted — make more of this.
信号传递显示资源在哪里被需要——多生产这个。
Incentive rewards producers so they want to supply more.
激励奖励生产者，使他们愿意供给更多。
Rationing shares a scarce good to those willing and able to pay.
配给把稀缺物品分给愿意且有能力支付的人。
In a pure market these three jobs replace a planner's order book.
在纯粹市场中，这三项工作取代了计划者的订单簿。
If the exam asks how resources are allocated without a central plan, name all three.
如果考试问没有中央计划时 资源如何配置，把三项都点出来。
Now the diagram this topic is really about.
现在来看这个专题真正的核心图形。
Imagine an economy making only two things — food and machines.
设想一个只生产两种东西的经济——粮食和机器。
The production possibility curve joins every combination it could make using all its resources fully and well.
生产可能性曲线连起了它在充分而有效地使用全部资源时所能生产的每一种组合。
Three points, three meanings, and marks for each.
三个点，三种含义，每一个都有分。
A point on the curve means every resource is used, and used well — that is efficiency.
曲线上的点表示每一份资源都被使用，而且用得好—— 这就是效率。
A point inside means something is wasted: workers unemployed, factories idle.
曲线内部的点表示存在浪费：工人失业，工厂闲置。
A point outside is impossible today.
曲线外部的点在今天做不到。
<slow>That point is scarcity, drawn.</slow>
那个点，就是把稀缺性画了出来。
Read the diagram the way the mark scheme does.
按评分标准来读图。
On the curve: full efficiency.
曲线上：充分效率。
Inside: waste — unemployment is the classic example, but idle machines count too.
内部：浪费——失业是经典例子，闲置机器也算。
Outside: unattainable with today's resources.
外部：以今天的资源无法达到。
Moving along the curve is a trade-off: more of one good means less of the other, because resources are already fully used.
沿曲线移动是取舍：多一种物品就少另一种， 因为资源已经被充分使用。
Move along the curve and you are looking straight at opportunity cost: to make more food you pull resources away from machines.
沿着曲线移动，你看到的就是机会成本本身：要生产更多粮食， 就必须把资源从机器那里抽走。
The shape tells you how that cost behaves.
曲线的形状告诉你这个成本如何变化。
A straight line means constant opportunity cost — every extra unit of food costs the same.
直线表示机会成本不变——每增加一单位粮食，付出的都相同。
A curve that bows outward means increasing opportunity cost — each extra unit costs more than the last.
向外凸出的曲线表示机会成本递增——每多一单位，付出的都比上一单位更多。
Side by side: a straight production possibility curve, and one that bows out.
并排放：一条直线生产可能性曲线，和一条外凸的。
Straight means constant opportunity cost — each extra unit of food costs the same number of machines.
直线表示机会成本不变—— 每多一单位粮食，付出的机器数量相同。
Bowed — concave — means increasing opportunity cost: each extra unit costs more and more machines, because factors are not perfectly substitutable.
外凸表示机会成本递增—— 每多一单位，付出的机器越来越多，因为生产要素并不能完全相互替代。
Move out of all-food production and the first factors you shift are the ones least suited to farming; keep going and you must move factors that are increasingly good at farming and increasingly bad at making cars.
从全部生产粮食开始往外移动时，你最先调走的是最不适合务农的要素； 继续下去，就不得不调走越来越擅长务农、却越来越不擅长造车的要素。
The bowed shape is the usual exam picture.
外凸形状是考试里常见的图。
Moving along the curve is a trade-off.
沿着曲线移动是取舍。
Moving the curve itself is growth.
让曲线本身移动，则是增长。
Push the whole curve outward and the economy can have more of both — from more resources, better technology, or a more skilled workforce.
把整条曲线向外推， 这个经济就能同时拥有更多——来自更多的资源、更好的技术，或者技能更高的劳动力。
It can also move inward after a war or disaster; that is decline.
曲线也可能在战争或灾难之后向内移动，那就是衰退。
And a choice hides here between capital goods and consumer goods.
这里还藏着一个在资本品和消费品之间的选择。
Consumer goods you enjoy now; capital goods make other goods, so building more today pushes the curve further out tomorrow.
消费品是你现在就享用的； 资本品用来生产其他物品，所以今天多造一些，明天曲线就会向外推得更远。
Economic growth is the whole curve shifting outward — more of both goods become possible.
经济增长是整条曲线向外移动——两种物品都变得更可能。
That comes from more resources, better technology, or a bigger, more skilled workforce.
它来自更多资源、 更好技术，或更大、技能更高的劳动力。
Economic decline is the curve shifting inward after war or disaster.
经济衰退是战争或灾难后曲线向内移动。
Do not confuse a move along the curve with a shift of the curve.
不要把沿曲线移动和曲线本身移动搞混。
Along is a trade-off.
沿曲线是取舍。
A shift changes what is possible.
移动改变的是可能性本身。
Capital goods are machines and factories used to make other goods.
资本品是用来生产其他物品的机器和工厂。
Consumer goods are food and clothes people use directly.
消费品是人们直接使用的食物和衣服。
Making more capital goods today means fewer consumer goods today.
今天多造资本品，就意味着今天消费品更少。
That feels like a sacrifice now.
这感觉像现在就要牺牲。
But more capital helps the economy grow faster, so the future curve shifts out further.
但更多资本帮助经济更快增长，所以未来的曲线会向外推得更远。
A country that eats all its output today grows more slowly tomorrow.
一个把产出全吃掉的国家，明天增长会更慢。
Not every good behaves the same way.
并不是所有物品的性质都一样。
A free good has no opportunity cost at all — supply is unlimited, so it has no price. Air, and sunlight.
免费物品完全没有机会成本——供给无限，因而没有价格， 比如空气和阳光。
An economic good is scarce, so it carries a price.
经济物品是稀缺的，因而有价格。
It is rival — if I eat the apple you cannot — and excludable, so the seller can keep you out until you pay.
它具有竞争性—— 我吃了这个苹果，你就吃不到了——也具有排他性，卖方可以在你付款之前把你排除在外。
A public good is the opposite on both counts: non-rival, and non-excludable.
公共物品在这两点上恰好相反：非竞争性，也非排他性。
Which creates the free-rider problem.
这就产生了搭便车问题。
No firm can profit, so the government must provide it.
任何企业都无法盈利，于是只能由政府提供。
An economic good is also called a private good when it has rivalry and excludability.
当经济物品具有竞争性和排他性时，也叫私人物品。
Rivalry means my use leaves less for you.
竞争性指我的使用留给你的更少。
Excludability means the seller can stop non-payers.
排他性指卖方可以阻止不付费的人。
A free good fails both tests of scarcity: unlimited supply, zero opportunity cost, no price.
免费物品两项稀缺检验都通不过： 供给无限、机会成本为零、没有价格。
Do not call air free if clean air becomes scarce — then it is an economic good.
如果洁净空气变得稀缺，就不要再叫它免费—— 那时它是经济物品。
Rivalry and excludability give four boxes, not two.
竞争性和排他性给出四个格子，不是两个。
Private goods are rival and excludable — food, clothes.
私人物品既竞争又可排他——食物、衣服。
Public goods are non-rival and non-excludable — street lighting and national defence.
公共物品既不竞争也不可排他——路灯和国防。
Club goods are non-rival but excludable — a cinema seat or a toll road.
俱乐部物品不竞争但可排他—— 电影院座位或收费公路。
Common resources are rival but non-excludable — ocean fish.
共有资源竞争但不可排他——海洋鱼类。
Public goods suffer the free-rider problem, so private firms will not supply them.
公共物品有搭便车问题，所以私人企业不会供给它们。
Why must the government provide street lighting and national defence?
为什么政府必须提供路灯和国防？
Because you cannot stop free riders.
因为你拦不住搭便车的人。
One more person under a street light does not use it up — non-rivalry.
多一个人走在路灯下并不会把它用尽——非竞争性。
And you cannot bill only those who look at it — non-excludability.
你也不能只向看灯的人收费—— 非排他性。
Firms cannot charge, so they will not supply.
企业收不到钱，就不会供给。
That is the free-rider problem in one sentence: people enjoy the good without paying.
这就是一句话里的搭便车问题： 人们享用物品却不付钱。
Two more, and they are about us rather than the good itself.
还有两类，它们说的是我们自己，而不是物品本身。
A merit good is better for you, and for those around you, than you realise — so people buy too little. Education, health care.
优值品对你、 以及对你周围的人的好处，比你以为的更大——所以人们买得太少，比如教育和医疗。
A demerit good is worse for you than you realise, so people buy too much. Cigarettes, alcohol.
劣值品对你的害处比你以为的更大，所以人们买得太多，比如香烟和酒。
Both come from the same fault: information failure.
两者都源于同一个毛病：信息失灵。
And each list is partly a value judgement.
而每一份清单，都带有价值判断的成分。
Education is the classic merit good.
教育是经典的优值品。
Its full value — better jobs later, and a more skilled society — is easy to underestimate.
它的全部价值——以后更好的工作，以及技能更高的社会—— 很容易被低估。
Left alone, people buy less of it than is good for them or the country.
放任不管，人们买到的会少于对自己或国家有益的量。
That is why governments provide and subsidise it.
这就是政府提供并补贴教育的原因。
Information failure is the root: people lack, or ignore, the full facts about benefit or harm.
信息失灵是根源：人们缺乏或忽视关于益处 或害处的全部事实。
Which goods count as merit or demerit is partly a value judgement, so lists can disagree.
哪些算优值品或劣值品，部分是价值判断，所以清单可以不同。
One exam question before we finish.
结束之前来看一道考题。
Why is a production possibility curve drawn bowed outwards, and not as a straight line?
为什么生产可能性曲线画成向外凸的，而不是一条直线？
Pause here and plan your answer.
先暂停，想好你的答案。
Ready?
好了吗？
The key phrase is that factors of production are not perfectly substitutable — they are specialised.
关键的一句话是：生产要素并非完全可以互相替代—— 它们是专门化的。
Start with all resources making food.
先假设所有资源都在生产粮食。
The first ones you move into cars are the worst at farming, so little food is lost.
你最先转去造汽车的那些要素， 正是最不擅长务农的，所以损失的粮食很少。
Keep going, and you must move factors better and better at farming.
继续下去， 你就不得不转移那些越来越擅长务农的要素。
So each extra car costs more food — increasing opportunity cost.
于是每多造一辆汽车， 付出的粮食就更多——这就是机会成本递增。
Now a number question.
现在是一道数字题。
An economy on its production possibility curve moves from sixty tonnes of food and thirty machines to fifty food and forty-five machines.
一个落在生产可能性曲线上的经济，从六十吨粮食加三十台机器， 移到五十吨粮食加四十五台机器。
What is the opportunity cost?
机会成本是什么？
Being on the curve means all resources are already fully and efficiently used.
在曲线上意味着资源已被 充分有效使用。
Gaining fifteen machines costs ten tonnes of food.
多得十五台机器，代价是十吨粮食。
That is the next best alternative given up.
那就是所放弃的次优选择。
Inside the curve you could gain both, so opportunity cost is really about moves along the frontier.
在曲线内部你可以两者都增加，所以机会成本真正说的是沿边界的移动。
A straight-line production possibility curve is not wrong.
直线生产可能性曲线并没有错。
It just assumes factors are perfectly substitutable — equally good at both goods.
它只是假设要素完全可以互相替代—— 对两种物品同样擅长。
Name that assumption and you earn the mark.
点出这个假设，你就得分。
The bowed shape is the usual picture because specialisation is real: the last factors you move are the ones best at the good you leave behind, so opportunity cost rises.
外凸形状是常见的图， 因为专业化是真实的：你最后转移的要素，正是最擅长你放弃的那种物品的， 所以机会成本上升。
Three habits that turn knowledge into marks.
三个把知识变成分数的习惯。
First, define opportunity cost as the next best alternative given up — not simply what you gave up. That one word carries the mark.
第一，把机会成本定义为「所放弃的次优选择」—— 而不只是「你放弃的东西」，就是这一个词决定得分。
Second, read the PPC properly: inside means waste, on the curve means efficient, outside means unattainable.
第二，正确地读生产可能性曲线： 内部表示浪费，曲线上表示有效率，外部表示无法达到。
Third, hunt for the word should — it marks a normative statement every time.
第三，去找「应当」这个词——它每一次都标志着规范表述。
Two more habits.
再加两个习惯。
Frame every long answer around scarcity and choice — that is the spine of this topic.
把每一道长答题都框在稀缺与选择周围——那是本专题的脊梁。
And classify goods carefully: free versus economic; private versus public; then merit and demerit.
并且仔细给物品分类：免费对经济；私人对公共；然后是优值与劣值。
Say non-rival and non-excludable for a public good.
对公共物品要说非竞争与非排他。
Say information failure for merit and demerit.
对优值与劣值要说信息失灵。
Vague labels lose easy marks.
含糊的标签会丢掉容易的分数。