语法与词汇 While they were away on vacation, they allowed their mail to _____ at the post office.

【正确答案】 A

【答案解析】句意：当他们去度假的时候，他们使得邮件在邮局堆积成山。 pile up堆积起来。 set up建立。 turn up出现。 mix up调和。因此，本题的正确答案为A。