题目内容：

They arrived here . A．since a week ago B．a week ago C．for a week D．next week
They arrived here .
A．since a week ago B．a week ago
C．for a week D．next week
They arrived here .
A．since a week ago B．a week ago
C．for a week D．next week
呵呵...请给出为什么选这个答案的理由.

优质解答

B．a week ago
They arrived here 时态是一般过去式,所以用过去的时间状语
ago通常做时间状语用一般过去
A．since a week ago 完成时
C．for a week 完成时
D．next week 将来时
这是选B