问题补充：
Zhangqiang is said the experiment in the lab, but I can’t find him there.
A. to have done
B. to do
C. to be doing
D. doing
答案：
A. to have done
B. to do
(答案→)C. to be doing
D. doing
解析：句意：有人说张强正在实验室做实验，但是我在那里没有找到他。sb be said to do sth 某人据说在做某事，本句中应该表示正在做某事，故选C
