Having checked the doors were closed,andall the lights were off,the boy opened the door to

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A. why
(答案→)B. that
C. when
D. where
解析：that引导宾语从句,在从句中只起连接作用,不作任何成分,常被省略,但当谓语后接两个或两个以上宾语从句时,第一个that可以省略,第二个that不能省略。故选B项。why引导宾语从句意为“为什么”。when引导宾语从句，意为“当……的时候”。where引导宾语从句意为“在……地方”。