单选题 I didn't know he came back ______ I met him in the street.
A. since B. when C. until

【正确答案】 C

【答案解析】考查连词固定搭配。not...until...是固定搭配，指“直到…才…”．符合句意逻辑。since引出从句，其前的语句应用完成时态,when引出时间状语从句，在此不合逻辑。