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

A COMMA + VERBing modifier serves to express CONTEMPORANEOUS action: an action attributed to the preceding subject and happening AT THE SAME TIME as -- and often as a result of -- the preceding action.
Thus, A implies the following:
When the students contribute $330 a year, they are AT THE SAME TIME previously paying $7 per year.
Not the intended meaning.

B states that students must contribute $330 a year toward their education, for which was previously paid $7 per year.
Here, it's unclear