问题描述：
..求详解为什么选
Tom _____ into the house when no one _____.
A.sliped ,noticed B.had slipped ,noticed C.slipped,had noticed D.was slipping ,was noticing .
Tom _____ into the house when no one _____.
A.sliped ,noticed B.had slipped ,noticed C.slipped,had noticed D.was slipping ,was noticing .
问题解答：
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