76. Buses run _________(频繁 ) between the city and the airport.

40.It was over a decade _____ someone else turned penicillin into the great drug of the 20^th century.

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

39. We must apply our minds to _____ the problem.

A. solve　　　　 B. solved　　　　　　 C. solves　　　　 D. solving

38.All of us are eager ______ success. The key ______ it is hard work.

A. to, to　　　　 B. for, to　　　　　　 C. to , of　　　　 D. of, for

37. The chemistry teacher was standing by the desk, explaining the exercises _____ the students had done at home.

A./　　　　　　　 B. to　　　　　　　 C. for　　　　　 D. with