【单选题】Suppose that for the past two months, you have studied economics one hour a day. You now decide to study economics two hours a day. For the past two months, ________. (假设在过去的两个月里,你每天都在学习经济学。你现在决定每天两个...
A.
your marginal cost of studying economics for an hour must have exceeded its marginal benefit你经济学一小时的边际成本必须超过其边际收益
B.
the marginal cost of studying economics must have fallen研究经济学的边际成本必须下降
C.
your marginal benefit from studying economics an hour must have been greater than its marginal cost小时学习经济学的边际效益必须大于其边际成本
D.
the opportunity cost of studying economics must have risen.学习经济学的机会成本必须上升。