NJC Surveys recently determined that, despite advances in audio technology, it is still ...(predominant/predominantly)... through radio broadcasts that listeners are exposed to new music.
The answer in my textbook is predominantly. Could you please explain?
The word in question modifies is, because rather than being all the time (that would be "it is still through radio broadcasts"), it is predominantly which basically means mostly or most importantly, but not always.
Since the word you are modifying is a verb, you need to use an adverb. In this case, predominantly.
