When I went into the office,the teachers were busy ____ preparing their lessons.(prep.)
问题描述：
When I went into the office,the teachers were busy ____ preparing their lessons.(prep.)
答
in
be busy doing sth.=be busy with sth.忙于做某事,
如：I'm busy with my homework.=I'm busy doing my homework.with后面接事（sth.）,没有with的话后面要接做什么事（doing sth.）.