21. ---Look! The telephone is broken. Someone damaged it ________ purpose.
---That may be right. But perhaps it was broken _________ accident.
A. on; by B. by; by C. on; on D. by; on
22．A ______ person always tries to finish the job, no matter how hard it is.
A. stubborn B. determined C. reliable D. remarkable
23. Was it in 2002 __________ he was still at middle school __________ the boy became expert at computer?
A. that; where B. when; where C. that; when D. when; that
