A)单项选择：从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21. -Look! The boys are playing with_______ unusual pet.
-_______ wonderful trick they are doing!
A. a, How B. a; What C. an; What a D. an; What
22. There are_______ workers in this factory. My brother is in it, too.
A. three thousand of B. thousands of
C. thousand of D. three thou
