-The frightening black clouds are gathering .lt .
-Sure Let's go indoors,
Ais to rain
Brains
Cis going to rain
Dis raining
网友回答
A. is to rain
B. rains
(答案→)C. is going to rain
D. is raining
解析：句意：上文，可怕的乌云正在聚集。马上要下雨了。下文，是的，让我们去室内吧。表示将要做某事，故用一般将来时，is going to rain指客观迹象表明将要发生某事，选C。