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。
例：It is generally considered unwise to give a child _____ he or she wants.
A. however B. whatever C. whichever D. whenever
答案是B。
21.- Good morning, Grand Hotel.
-Hello, I'd like to book a room for the nights of the 18th and 19th.
-_______
A. What can I do for you? B. Just a minute, please.
C. What's the matter? D. At your service.
