Sarah knew nothing about it her sister told her. A.if B. until C. since

问题补充：

Sarah knew nothing about it her sister told her.

A.if B. until C. since

答案：

解析：if如果，是否；until直到； since由于，自从。句意：直到她的妹妹告诉她，她什么都不知道。故选B。