屏幕上的问题，框给 AI。

Opportunity cost is the value of the next best alternative you give up when you make a choice.机会成本，是你做选择时放弃掉的、次好的那个选项的价值。

If you spend Saturday studying, the cost is not only the coffee you buy, but also the paid shift you could have worked.如果你周六用来学习，代价不只是买的那杯咖啡，还有你本可以去上的那一班带薪工作。

Economists use this idea to explain why people respond to incentives: when the reward for one option rises, its alternatives become more expensive.经济学家用这个概念解释人为什么会对激励做出反应：一个选项的回报变高，其他选项就变得更“贵”。

A production possibilities frontier shows every combination of two goods an economy can make with its current resources.生产可能性边界，画的是一个经济体用现有资源能生产的两种商品的所有组合。

Points inside the curve mean resources are left idle; points outside it cannot be reached without new technology or more inputs.曲线以内的点说明资源有闲置；曲线以外的点，没有新技术或更多投入就达不到。

Moving along the curve always involves a trade-off: producing more of one good means producing less of the other.沿着曲线移动一定伴随取舍：多生产一种商品，就得少生产另一种。

The curve usually bows outward because resources are not equally good at producing both goods.这条曲线通常向外凸，因为资源生产两种商品的效率并不一样。

When a country specializes in the good with the lower opportunity cost, both trading partners can end up with more.一个国家专门生产机会成本更低的商品，贸易双方最后都能得到更多。

This is the idea of comparative advantage, and it explains why trade can benefit even the less efficient side.这就是比较优势，它解释了为什么连效率低的一方也能从贸易中获益。

Summary: every choice has a cost, and the true cost is what you give up.小结：每个选择都有成本，真正的成本是你放弃掉的东西。