TOEIC・English Undergraduate almost 4 yearsago

この問題でaとcで迷ったんですが、なぜaではだめなのですか。 よろしくお願い致します

31. H ------- is considered highly effective, but remains S&HORCAJ Std stiz O sol a'blousH y 40 +1 Word-of-mouth a rarely studied phenomenon. (A) advertise (B) advertised (C) advertising (D) advertisements X30