she is not so cheerful she used to be?
A. How it is that；as
B. How is it that；as
C. Is it why；that
D. Why is it that；what
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A. How it is that；as
(答案→)B. How is it that；as
C. Is it why；that
D. Why is it that；what
解析：考查强调句的特殊问句和比较级：强调句的特殊疑问句式是：特殊疑问词+is/was it that+其他部分？句意：为什么（怎么）她不像以前一样那么高兴，像…一样 === as ...as（陈述句） === so ...as（否定句）,选项D中的what不能和前面的so构成同级比较，因此B项正确。