从A.B.C.D四个选项中，选出可以填入空白处的最佳选项.
21. ______she couldn’t understand was _______ fewer and fewer students showed interest in her lesson.
A. What, why B. That, what C. What, because D. Why, that
22. You should make it a rule to leave things＿＿＿＿＿you can find them again.
A. when B. where C. then D. there
23. Why do you want a new job _____ you've got such a good one already?
A. that B. where C. which D. when
