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
24. He was badly injured and couldn’t _______ the game.
A. attend B. join C. participate D. take part in
