边际替代率
参考答案：
指如果消费者在收到额外一单位商品X之后要保持满足水平不变而必须放弃的商品Y的数量。
—Didn't you finish your homework last night?
—_______, though I wasn't feeling well.[ ]
A. No, I hadn't
B. Yes, I had
C. No, I didn't
D. Yes, I did
当沉淀池水面冒出气泡时，表明池底污泥已经腐败。
