_________ my dear teacher couldn’t understand was _________ fewer and fewer middle school students showed interest in her lessons．
A.What; whyB.That; whatC.What; becauseD.Why; that在线课程A
整个句子的意思是“老师不明白为什么越来越少的学生对他的课感兴趣”。was 之前的部分是主语从句，由what引导，并作主语；was之后的部分是表语从句，由why引导。答案是A项。