________ is no doubt that he will come here some day, but ________ is doubtful when he will come.
A.There; thereB.It; itC.There; itD.It; there在线课程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．