c
It is （high） time that ,后面的从句谓语动词要用过去式或用should加动词原形,但should不可省略.例如：
It is time that the children went to bed.
孩子们该睡觉了.
It is high time that the children should go to bed.
这个题目省略了that.
翻译：圣诞假期你要做什么?不知道,该是我决定的时候了.