They have been friends _________

试题详情及答案解析

They have been friends __________ they were at primary school.

A．before	B．as soon as
C．since	D．when

答案：C

试题分析: 句意：自从他们上小学就已经成为朋友了。此题考查现在完成时since是现在完成时的标志词；根据句意，故选C。
考点：考查连词。