that he had fallen behind,he tried to work harder.
A. Having realized
B. Realized
C. Realize
D. To realize
网友回答
(答案→)A. Having realized
B. Realized
C. Realize
D. To realize
解析：考查非谓语动词。句意：由于意识到他已经落后了，他尽力努力工作起来。现在分词做状语表示主语正在进行的伴随性动作，如果动作发生在谓语动作之前，则要用完成时态的现在分词。结合语境可知意识到落后这个动作发生在尽力工作之前，故选用完成时态的现在分词。选A。