一道unless题

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.......

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