GMAT Sentence Correction (SC) | Re: Most of Portugals 250,000 university students boycotted

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 ayear 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