As soon as I got on the bus I realized that I my papers in the house.A. leftB. was leavin

问题补充：

As soon as I got on the bus I realized that I my papers in the house.

A. left

B. was leaving

C. had left

D. have left

答案：

A. left

B. was leaving

(答案→)C. had left

D. have left

解析：考查时态。句意：我一登上公交车就意识到我把我的论文丢在家中了。结合语境可知从句描述的是过去已经完成的动作，故用过去完成时态，选C。