Billie jean has $120 to spend and wants to buy either a new amplifier for her guitar or a new mp3 player to listen to music while working out. both the amplifier and the mp3 player cost $120, so she can only buy one. this illustrates the basic concept that:
a) trade can make everyone better off
b) people face trade offs
c) rational people think at the margin
d) decisions made at the margin are not particularly important

B) people face trade offs

Explanation:

Resources are scarce, and that applies to every person and every organization in the world. Even the richest person or richest organization has a certain finite amount of money, time and other resources. For example, if we decide to work 12 hours a day, we will make more money, but we will also have less time o do other activities that we like.

In this case, Billie Jean only has $120 and she wants to buy both products, but she can only buy one. Whatever product she decides to buy will leave her with $0, so if she wants to purchase the other product she will need to find a job and earn some money, or if she already has a job, she will need to work more hours.

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