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. — Sally, was ________ problem solved?
— We were trying to think of ________ way out, but it was hard to find one.
