单选题 While they were away on vacation, they allowed their mail to ______ at the post office.
【正确答案】 B
【答案解析】[解析] 句意：当他们度假外出时，他们允许信件堆积在邮局。根据地点状语at the post office可知信件还是要放在邮局，pile up意为“堆积”。故选B。