问题补充：
Everyone can borrow from this public library only two books and they must be returned within a month.
A. at one time
B. at times
C. at the time
D. at a time
答案：
A. at one time
B. at times
C. at the time
(答案→)D. at a time
解析：本题的含义是每个人每次可以在图书馆借2本书，他们必须在一个月内归还。at a time可以表示一次或每次的意思，故本题选D。
