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

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

we've been friends_____we came this school a year agoA 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.

D since 自从

since自从
意思为自从一年前我们进入这所学校时就成为了朋友

D

d

D

D