21. Sigma Tel Inc. , _____ world’s largest maker of controller chips for MP3 players, is set to establish _____ new R & D centre in Hong Kong and open two sales offices on China’s mainland.

A. the; a                     B. the; the                  C. a; the                     D. a; a

22. —He is a very hard-working student.

—_____ I know, he often burns the midnight oil.

A. As far as                B. As well as              C. As long as             D. As soon as

23. He said he would _____ me to Professor Mcrae as his postgraduate student.

A. comment                                B. suggest                 

C. command                               D. recommend

24. The main cause of HIV / AIDS infection in China is drug abuse, _____ is very serious in Yunnan.

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