问题补充:
—Must we keep the room all the time?
—No, you don’t have to.
A. cleaning
B. cleaned
C. to clean
D. clean
答案:
A. cleaning
B. cleaned
C. to clean
(答案→)D. clean
解析:keep sb/sth +adj让某人或某物保持某种状态。根据句意:我们必须一直保持教室干净吗?--不,你不必。 keep the room clean让教室保持干净,故选D。