predominant or predominantly

[hoangkha]

Hi!
It is ______ through broadcast media such as television and radio that companies place an advertisement to attract prospective customers.
a. predominant
b. predominantly
c. predominanting
d. predominanted
I have chosen A because I think it is the construction of IT + BE + Adj + THAT-CLAUSE, but the key is B. Could you please explain that?

 

[probus]

An adverb is required here. You can see that by simplifying the construction: companies place advertisements predominantly through broadcast media. The construction you propose does not exist.