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

问题补充：

Tom realized that he his physics book at home when he reached school．

A. left

B. had left

C. was leaving

D. leaves

答案：

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。