she couldn’t understand was fewer and fewer students showed interest in her lessons.
A. What；why
B. That；what
C. What；because
D. Why；that
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(答案→)A. What；why
B. That；what
C. What；because
D. Why；that
解析：考查名词性从句。what she couldn’t understand是主语从句，what引导这个主语从句并在句中作宾语。后面是一个表语从句，句子结构很完整所以应该使用连接副词，再从句意考虑，使用why。句意：她不能理解的是为什么越来越少的学生对她的课感兴趣。