Since Solano Marketing was established,the human resoureces department has counted on newspapers to advertise job opentings in the company.However,we have observed that ____ to printed advertisements have dropped since the onset of online employment agencies.
A. responses B.responded C.responding D.respondent
答案是 A,为什么是 A,为什么不是 C,respond to 回应...,所以不是应该选C吗?
因为是语法初学者,麻烦能不能详细回答下呢?