[求助] 毫无头绪的一道gwd语法题

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

这道题目答案是D，我选的是B。但其实除了C以外（previously是副词，不能修饰$7），我都不知道其他答案哪里有错。请哪位好心的NN赐教！