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。
