________ is no doubt that he will come here some day, but ________ is doubtful when he will come.

A.There; there

B.It; it

C.There; it

D.It; there

答案及解析： C

解析：

导解：第一句为there be句型，主语是名词doubt；

第二句为it作形式主语，doubtful是形容词作表语．