题目内容：
What is (21) that a teacher most wants in his students? Attentiveness? A good memory? Diligence? Certainly these are the qualities commonly (22) with "good students" in the (23) mind. And certainly, too, these are the qualities that most (24) to teachers' comfort.But the best students I ever had, (25) I remember the most wistfully, was a talkative, lazy day-dreamer. Sometimes he turned assignments in (26) , and a few he never (27) around to doing at all. Actually, my admiration for him was ironic, (28) I have never liked the name Ronald, (29) suggests to me the assumed name of a movie star. But he made one whole year of my (30) experience a delight.
21.
A. there
B. they
C. it
D. that
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