问题补充:
When he home, he saw his mother the room.
A. got back; clean
B. got on; cleaning
C. got at; clean
D. got back; cleaning
答案:
A. got back; clean
B. got on; cleaning
C. got at; clean
(答案→)D. got back; cleaning
解析:考察时态,根据题意,当他回到家时,发现他的母亲正在打扫房间。可知时间状语的动作发生在过去,动名词表示正在发生的事情,伴随性的动作。故选D。