题目内容：

21.______ we know more and more about George,we find that he is a man who can not be trusted.
A.Then B.If C.Unless D.As
22.I finished the job on August 28th,but ______ a very interesting event had taken place.
A.meanwhile B.meantime C.meanings D.mealtime
23.The gentleman over there told me that he was not living here and he was only here ____.
A.in a visit B.for a visit
C.with a visit D.on a visit
26.If I ______ how difficult the experiment was,I would never have taken part in it.
A.knew B.know C.did know D.had known
27.If you stay in this hotel,you should know that dinner costs $5,and wine is ______.
A.extra B.extent C.extensive D.extract
28.When the teacher came into the classroom,all the students stopped ______.
A.to talk B.and talk C.and talked D.talking
29.This is regarded by the government as a possible ______ to national security.
A.thread B.threat C.thrill D.thrift
30.Police have _____ the area for about two days,but so far have found nothing.
A.searched B.looked C.found D.discovered
31.When she took her seat,Emma opened her bag and ______ her comb.
A.took out B.took up
C.took over D.took off
32.They have bought a new house and are going to decorate it ______ cost.
A.because of B.regardless of
C.at the risk of D.in respect of
33.We must stop ___ the earth’s resources,because there are less and less things we can use to survive.
A.filling B.wasting C.changing D.giving off
34.I don’t want to _____ anyone by saying that these two words are almost the same.
A.mislead B.misprint C.mistake D.misuse
35.______,I don’t think we have a chance of winning.
A.To be honest B.Being honest
C.Be honest D.To being honest
36.I really think it ______ to read the personal handwritten letters by Edward Snow again and again.
A.pays off B.pays out
C.pays back D.pays over
37.Once you have cured the water pollution problem,you will have to take steps to avoid it _____ again.
A.to happen B.happens C.happening D.happen
38.To our delight,the argument had finally ____ into a friendly discussion.
A.rose B.raised C.came D.rolled
39.A couple of new products have recently _____ in the market,and so we can have more things to choose.
A.arisen B.appeared C.emerged D.awaken
40.When a policeman have seen the bus driver running a red light,he called to him to _____ and walked towards the bus.
A.pull in B.pull over
C.pull out D.pull into