21. You may use the room as you like ______ you clean it up afterwards.
A. as far as B. so long as C. in case D. as if
22. People in China are proud of ______ they have achieved in the past few years.
A. what B. which C. that D. how
23. Gun control is a subject ______ Americans have argued for a long time.
A. of which B. with which C. over which D. into which
24. ______ the Internet is of great help, I don’t think it’s a good idea to spend too much time on it.
A. If B. As C. Because D. While
