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. Only when everybody began to attack the dirty conditions _________ and was not a problem any more.
A. was cholera defeated B. cholera was defeated
C. did cholera be defeated D. be cholera defeated
22. The results of the scientific experiments are valuable to us and they will _______ the success of our research.
A. contribute to B. stick to C. expose to D. guide to
