从每题所给的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. Earth is a rocky planet _______ liquid water, which is necessary for life, can exist.
A. as B. which C. where D. when
22. After he retired from the office, Tom _______ painting for a while, but soon lost interest.
A. took up B. saved up C. kept up D. drew up
23. Such women _____ know Tom thought he was charming.
A. that B. which C. / D. as
