551.I was giving a talk to a large group of people,the same talk I ______ to half a dozen other groups.
A.was giving
B.am giving
C.had given
D.have given
552.Police are now searching for a woman who is reported to ___ since the flood hit the area last Friday.
A.have been missing
B.have got lost
C.be missing
D.get lost
551.答案是C;给其他六个组做报告发生在给这一大组人做报告之前，故用过去完成时态。
552.答案是A;由句意得知:失踪这一动作已经发生，故须用完成时。
