20. When ____ comes to repairing a machine, I have little knowledge of it.

　 A. that　　　　　　　　 B. one　　　　　　　　 C. this　　　　　　　　 D. it

19. Try to ____ at least half an hour each day for learning new vocabulary, and you’ll know more words.

　 A. set aside　　　　　　　　　 B. take up　　　　　　　　　 C. put away　　　　　　　 D. turn out

18. Creating world peace is a rather difficult goal, ____ calls for efforts of many generations.

　 A. one that　　　　　　　　　　　　　　 B. one　　　 　　　　 C. that　　　　　　　 D. it