请教：GWD29-Q29

40: GWD-29-Q29

Most of Portugal’s 250,000 university students boycotted classes in a one-day strike to protest a law that requires them to contribute $330 a
                year toward the cost of higher education, previously paying $7 per year.

A.      year toward the cost of higher education, previously paying $7 per year

B.       year toward the cost of higher education, for which was previously paid $7 per year

C.      year, compared to the previously $7 per year, toward the cost of higher education

D.      year toward the cost of higher education, instead of the $7 per year required previously

E.       year as opposed to the $7 per year required previously for the cost of higher education

请教一下各位高人，答案为什么是D呢？B错在哪里呢？which指代cost不行吗？

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