A policeman saw two thieves____(steal)a girl's mobile phone on a bus and he caught them at once
问题描述：
A policeman saw two thieves____(steal)a girl's mobile phone on a bus and he caught them at once
答
这里应该用进行时,stealing
see sb.doing sth.看见某人正在做某事
这里是警察看见了有俩贼正在偷一个美眉的手机,于是立马逮住他们了.所以是过去进行时.