What do you think ---left in his classroom?

B
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在特殊疑问句中如果有插入语you think ,那么提前you think的主动词do,剩下的是陈述语序
wh-疑问词 + do you think + 陈述语序。
如：
who do you think will be our monitor?
where do you think he came from?
which do you think he likes?
why do you think he told a lie?
whom do you think he will give the gift to?
How do you think he solved the problem?
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其次B中的he‘s=he has。现在完成时
其实这里的关键词是left
这里不是被动语态