we were busy ____the classroom when mr wang came in 1.with clean 2.cleaning 3.to clean 4.for
问题描述：
we were busy ____the classroom when mr wang came in 1.with clean 2.cleaning 3.to clean 4.for
答
选项应该是2,
词组结构为 be busy doing sth,忙于做某事,这是一个基础结构,也属于习惯用法