. that he had wasted a lot of time Tom decided to work hard on his study.ARealizingB.

问题补充：

. that he had wasted a lot of time, Tom decided to work hard on his study.

ARealizing

B. To realize

C. Realized

D. Having been realized

答案：

(答案→)A. Realizing

B. To realize

C. Realized

D. Having been realized

解析：句意,由于汤姆意识到自己已经浪费了很多时间，所以他决定好好学习。非谓语动词作状语，主要看非谓语动词和主语的关系。现在分词作状语多表示主动,正在进行的动作,过去分词作状语，表示被动和完成.不定式作状语通常表未来，汤姆和意识到是主谓关系，故用现在分词作状语，表原因，故选A。