Sometimes children have trouble ________ fact from fiction and may believe that such things actually exist．

A.to separate

B.for separating

C.separating

D.of separating

答案及解析： C

句意：有时候孩子们很难把现实与虚构区别开来，可能认为那样的事真的存在。此题用have trouble/difficulty in doing句式，介词in可省略。