一道unless题

huangluyaomm

A package is never accepted for delivery by the delivery service unless it is within the established size limits. All packages accepted for delivery by delivery service have a return address.
If the statements above are true, which of the following must also be true?
A  The delivery service charges more for heavier packages that for lighter packages.
B The delivery service will always accept for delivery a package that is within the established size limits.
C If a package is within the established size limits and has a return address, it will be accepted for delivery by the delivery service no matter how heavy the package is.
D A package that is not within the established size limits but has a return address is never accepted for delivery by the delivery service.
E The delivery service does not charge for package that must be returned to the sender.
答案是D. 我选了B.因为这个就是文中的逆否啊.......

fangzep

我觉得文中的逆否应当是：若包裹被接受了，包裹一定符合大小限制。
但是B的表达是说，只要包裹符合了大小限制就一定会被接受。这两个表达是不一样的。

xiangzhengj

貌似：

原句意思within the size 是deliver的充分条件

B变成了within the size是deliver 的必要条件

lengling

b当中 within the size就是充分条件吧，ls？

besttys

B明显不对：题干：接受邮包推出邮包SIME对，B将之反过来是推不出来的。