问题描述：
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.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",
问题解答：
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