we've been friends_____we came this school a year ago

问题描述:

we've been friends_____we came this school a year ago
A when B for C before D since
选哪个才对呢?
为什么BEFORE 不对呢?

D since
一年前我们来到这所学校,就成了朋友.
不用before的原因:have been强调一年前我们来到这所学校这个时间段,如果用before意思就成了我们来学校之前就已经是朋友,have been应该改成过去式were.