请教一道语法题（PP－1－8）－Zeros修改标题，见谅

In instrumentalist philosophy, ideas and knowledge are exclusively functional processes: they are of significance only____________ instrumental in the development of experience.

A. AS THEY ARE

B.ARE THEY

C.THERE ARE

D.ARE

WHY CHOOSE  A, DON'T GET IT? NN PLS HELP ME! THANK YOU!