She looks sad.Could you please tell me ____that prevents her form_英语解答

She looks sad.Could you please tell me ____that prevents her form being as happy as before?
a.what it is
b.it is what
c.how it is
d.it is how
+原因
老师说:"被包含时一般用陈述句",

人气：295 ℃　时间：2019-08-20 21:53:05

解答

A.
你可以把从句单独拿出来分析,如果是单独的句子,就是what prevents her .,所以肯定不是HOW,然后that是代词做从句的主语的.B的话IT也是形式主语,两个主语肯定不对嘛.
或者你可以把what it's that 看成做从句主语.