_______she couldn’t understand was _______fewer and fewer studentsshowed interest in her lessons．
A.What；whyB.That；whatC.What；becauseD.Why；that在线课程A
解析：
主语从句shecouldn’t understand缺少宾语，要用what引导。表语从句fewer and fewer students showed interestin her lessons是一个完整的句子，要用why引导，意思是“为什么越来越少的学生对她的课感兴趣”。