A
I don't think Jack will come today 中的宾语从句本来是：Jack won't come today.I think相当于插入语,它把谓语中的否定词 not 前移了.做题时,我们在思想中应当去掉插入语I think,将not 还原：
---Jack won't come today.
---Nor will Mary.
甲没做某事,要表达乙也没做某事,就要用Neither/ Nor引起的倒装句.
B项除了没用neither/nor引导,没用倒装句,还有一个问题,就是时态与上一句不一致：上一句是一般将来时,而B项用了一般现在时.