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52._____ private cars are bringing us convenience, they may also cause more traffic accidents and pollution.

A.While　　 B.As　　　 C.If　　　 D.Since

51. Doing your homework is a sure way to improve yor test scores, and this is especially true ____ it comes to classroom tests.

A. before　 B. since　　 C. when　 D. after

50. So many uses have been discovered for this wood ____ the supply of white pine is becoming smaller and smaller.

A. then B. what　　 C. that　 D. which

49. I wrote to my parents once a week,____ I was too busy with my teaching work.

A. it　 B. unless　 C. when　　 D. as

48.I wonder ____ we can solve this hard problem.

A. what else　 B. how else　 C. that　　 D. as if

47. Mr Smith is ______ a good teacher ______ we all respect.

　A. such；that B. such； as

　C. so；that D. so； as

46. Wang Ling was elected ______ all he is the tallest.

　A. because B. because of

　C. for D. as

45. - Was it under the tree ______ you were away talking to a friend？

　- Sure. But when I get back there， the bike was gone.

　A. that 　　B.where C. which D. while

44. 　He will tell you _____ he expects will win such a match.

　A. why 　B. whom 　C. which 　　D. who

43. 　We should do more such exercises in the future， I think， _____ those we did yesterday.

　A. as 　　B. like 　C. about 　　D. than