－Sir may I borrow some new books?－Sorry you can'tyou haven't turn back those you borrowed

问题补充：

－Sir，may I borrow some new books?

－Sorry，you can you haven turn back those you borrowed one month ago.

A. unless

B. before

C. because

D. until

答案：

A. unless

(答案→)B. before

C. because

D. until

解析：C考查连词。根据答语“对不起，你不能，因为一个月之前借的那些书你还没有还。”可知答案。