The teacher won't explain

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选A,名词性从句,C,D可以直接排除,从句应该用陈述句语序；The teacher won't explain (that)what it is that makes his students so fascinated with his lessons.这个是完整的句子,第一个that 可以省略,第二个省略的话表语从句主语不可以省略.
一年多没接触语法了,但是我觉得自信还是有点的,
======以下答案可供参考======
供参考答案1:
A.what it is that
供参考答案2:
Awon't explain+宾语从句what it is
that 定语从句修饰what