为什么选a? 我选了b，因为shoes可以多买几双。。。MRS of .. for中文是什么？

20.       Suppose that an individual has a constantMRS of shoes for sneakers of  :(that is, he or she is always willing to give up 3 pairs of sneakers to get 4pairs of shoes). Then, if sneakers and shoes are equally costly, he or she will

*        a.         buyonly sneakers.

b.         buy only shoes.

c.         spend his or her income equally on sneakers and shoes.

d.         wear sneakers only 3/4 of the time.