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

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",

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.

B.It is sth that do sth. 实际上等于sth do sth.但是这里强调了做事的一方。
所以本句也可以写成...tell me what prevents....

a
后面的句子缺少的是 主语，所以前面填的应该是what，但是前面的句子缺少的宾语，所以该是A