Answer: b. people face trade-offs
Step-by-step explanation:
From the question, we are informed that Bullie Jean has $120 to spend and wants to buy cither a new amplifier for her guitar or a new mp3 player to listen to music while working out.
We are further informed that the amplifier and the mp3 player cost $120, each and so she can only buy one. This shows that people face trade offs and have to make a choice regarding some decisions. Here, an opportunity cost will be the one that she didn't buy at the expense of the other.