39.   Rules banning cancer-causing substances from food apply to new food additives and not to natural constituents of food because their use as additives is entirely avoidable.

(A) their use as additives is

(B) as additives, their use is

(C) the use of such additives is

(D) the use of such additives are  （C）

(E) the use of them as additives is

答案是C,可是我怎么就看不出句子结构来呢？好像只有主语呀，请赐教！