（）in the queue for half an hour,tom sud denly realized that he had left his wallet at home
问题描述：
（）in the queue for half an hour,tom sud denly realized that he had left his wallet at home
a:To wait b:Have waited c:Having waited d:To have waited
为什么要用having?b,
由tom做的动作所以用ing形式?表主动?、
请一个一个的解释
答
这是个非谓语结构,真正的谓语动词是 realize ,如果表示主动则为 Having done,被动则为 Having been done,意思是后面的这个动作在谓语动词表示的动作之前发生,本题中wait 动作发生在realize前,并且是Tom 主动发出的,故为正解.C 选项这种结构是不存在的,没有to + 完成时这种结构.