what must you do before you return a book to th library

问题描述:

what must you do before you return a book to th library

是翻译吗?
在你把书归还图书馆之前必须要做什么?

呵呵,这是脑筋急转弯啊!请看答案:
—What must you do before you return a book to the library?
--You must borrow it.