英语语法独立结构All the tasks —— ahead of time ,they decided to go o

All the tasks —— ahead of time ,they decided to go on holiday for a week .
A been fulfilled B were fulfilled C having been fulfilled D had been fulfilled
答案是c 为什么
译：由于所有的任务都已经提前完成,所以他们决定去度假一周.
析：主句在后,逗号前面的不是一个句子,应用非谓语动词作状语的,但是“被完成的”不是“they”,现在分词完成被动式having been fulfilled带有自己独立的逻辑主语All the tasks,这就叫独立主格结构,简称独立结构,这你知道的.
BD.是谓语动词.A.是错误的结构,既不是谓语动词形式,也不是非谓语动词形式.