Someone had told the police that thieves would try to steal the diamonds.句意为:有人告诉警察有小偷会去偷这个宝石.This happened______the plane arrived. 首先明确this指代的是"有人告诉警察有小偷会去偷这个宝石"这件事,所以后半句句意应为:这件事发生在方案到来之前.这是因为在第一个句子中出现了过去完成时,说明先是有人告诉警察了这件事,然后方案才来的,故用before,选A这是我的个人推理,仅供参考