she couldn’t understand was fewer and fewer students showed interests in her lessons.A.

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(答案→)A. What; why
B. That; what
C. What; because
D. Why; that
解析：考察连词。根据句子结构可知，第一空为主语从句，从句中缺少understand的宾语，第二空为表语从句，从句中缺少原因状语。
句意为，她无法理解的是，为什么越来越少的学生对她的课感兴趣。