__________ experiments on the laboratory, it started to rain heavily.
A、While performing
B、Performing
C、While I was performing
D、To perform
【正确答案】：C
【题目解析】：句意：当我正在实验室做实验时，天开始下大雨了。While【 I 从句主语】 was performing experiments on the laboratory ,【it 主句主语】started to rain heavily。即，主句和从句主语不一致，那么，各自的主语必须分别而出现，而不能用现在分词等非谓语动词形式，因此，A,B,D都可以排除。
