21. The Niagara Waterfalls, __________ is shaped like a semicircle, are 670meters wide, and fall 56 meters in an awesome white sheet of water.

A. what            B. where           C. the largest of which      D. in which

22. __________ all my homework unfinished, I am not allowed to go online to play games.

A. To            B. For                 C. With                 D. Without

23. How can you expect to pass the Oral Test of PETS _________you never dare to practice in public?

A. in case         B. even if              C. unless                D. if