This is and we must on it.A hard work; work hard B a hard work; work hardC hard work; ha

问题补充：

This is and we must on it.

A hard work; work hard B a hard work; work hard

C hard work; hard work D work hard; hard work

答案：

解析：句意：这是艰苦的工作，我们必须努力工作。Hard work艰苦的工作；work hard努力工作。英语中work为不可数名词，故选A。