When I went into the office,the teachers were busy ____ preparing_英语解答

When I went into the office,the teachers were busy ____ preparing their lessons.(prep.)

人气：334 ℃　时间：2020-04-24 01:03:21

解答

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.）.