30. Not until he met with the difficulty _______ the importance of group work.

A. he realized　　　 　 B. did he realize 　　　　 C. realized he　 　　　　　　 D. he didn’t realize

29. Mary got the first prize in the National Math Competition, ________, for sure, delighted everyone.

A. who 　　　　　　　　　　　 B. that 　　　　　　　　　　　　 C. this 　　　　　　　　　　　　 D. which

28. ______ he had broken the record, he would be a world-famous runner now.

A. When 　　　　　　　 　　　 B. Because　　 　　　　　 C. If　　　 　　　　　 D. Though

27. -Was the driving pleasant when you vacationed in Mexico last summer?

-No, it _______ for four days when we arrived, so the roads were very muddy.

A. has rained　　　　　　　　 B. had been raining　　　　　　　 C. rained　　 　　　　　　　 D. has been raining