21.The bird ______ its wings and flew to the south.
A. beat B. hit C. strike D. struck
22. Dr. Frank failed many times but he finallyto find a successful way to solve the problem.
A. broke off B. broke up C. broke down D. broke through
23. The class is unusual _______ the students are all over fifty and none of them attended the middle school when they were young.
A. in how B. in what C. in which D. in that