Tom realized that he his physics book at home when he reached schoolA. leftB. had leftC.

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A. left
(答案→)B. had left
C. was leaving
D. leaves
解析：
本题考查过去完成时的用法，由had+过去分词。我们可以看到意识到和到都是过去的动作，而忘记带物理书则是在意识到之前，故为过去的过去，选B。