GWD29-29

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.

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

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

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

4. year toward the cost of higher education, instead of the $7 per year required previously
5. year as opposed to the $7 per year required previously for the cost of higher education

答案是D,为什么B不对. which指代the cost
  

我弱弱的问一句，C为什么不对？

compared to是比喻的意思,即把A比作B. A和B不同;比较要用compare with,即把A和A'的不同点区分开,A和A'是同类.

还是没明白, which指代the cost of higher education啊.

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

我的想法是B这里错不是因为定语从句缺主语,而是没有表达出比较的含义.

B选项没有correctiveness错误，但effectiveness错误，个人觉得还不少。

1，用被动，使得动作的发出者不明。

2，句式上的不对称没有明显的表现出前后的对比。

3，which优先指代cost，但实际上which指代higher education会更简洁明确一些。当然这一点不能作为排除选项的理由。

我不明白

为什么要选B？

for怎么解释呢？

for which中的for和从句中那个介词短语相关呢？