amount 和 quantity 的区别

amount 和 quantity 的区别
If you can afford only a small ____16 of health insurance, it is best to insure yourself____17 the big unexpected costs and pay the smaller common bills yourself.
16. A. number B. amount C. figure D. quantity
17. A. against B. for C. of D. on
16题答案为 amount。为什么不能选择quantity，请详细说明，谢谢
这是自学考试英语二的考题