-Was his father very strict with him when he was at school?
-Yes.He had never praised him ___ he became one of the top students in his grade.
A.after B.before C.until D.unless
请详细解释一下。谢谢！

B
当他在学校（上学）时，他父亲对他是不是要求很严格？
是的，在他成为全年级的尖子生之前，他从没表扬过他。

until 常常俩边都用过去时。或主将从现原则。

1. 直到...时,到...为止
He worked in Hong Kong until his father found him a job in Canada.
在他父亲给他在加拿大找到差事前,他一直在香港工作。
2. (用于否定句)在...以前,直到...才
She didn't go to bed until her daughter came back.
她一直等到女儿回来才去睡觉。