问题补充：
When the teacher came in, he saw some boys behind Daniel, the computer games.
A. sit, watching
B. sitting, watching
C. sit, to watch
D. sitting, watched
答案：
A. sit, watching
(答案→)B. sitting, watching
C. sit, to watch
D. sitting, watched
解析：第一空考查的是see sb doing sth，表示看见某人正在做某事，后一空为动词的现在分词作时间状语，故选答案为B
