请从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该选项的标号涂黑。
例：A computer can only do ________ you have instructed it to do.
A. how B. after C. what D. when
答案是C。
21. In Sydney, there’s only ________ average of 23 days a year when ________ sun doesn’t shine.
A. the, the B. an, / C. an, the D. the, /
22. –You look so angry. What happened?
–_______ I’d rather not talk about it.
