请教新东方书上的一道语法题！！

selenaly

Most of Portugal's 250,000 university studnets 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.

(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

答案是d，为什么b, c不对呢？

margaret1970

(C)compared to 是不同事物的比较,应是compared with.

(B)没有单词引出比较,并且 which 引导的从句缺主语