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标题: 一道unless题 [打印本页]

作者: xiaoxique89 时间: 2009-12-20 21:48 标题: 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.......

作者: woodlittle 时间: 2009-12-21 06:54

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。

作者: lindar1009 时间: 2009-12-21 20:43

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

作者: for120 时间: 2009-12-21 21:31

A package is never accepted for delivery by the delivery service unless it is within the established size limits.

   Conclusion A. if not S(size Ok), not A(accept)=〉[ if A, then S]

All packages accepted for delivery by delivery service have a return address.

   Conclusion B. if A, then R(have return address)

Choice B The delivery service will always accept for delivery a package that is within the established size limits

               =[ If S, then A]

Choice D [if (not S)&R, then not S. ------If not S, then not A]

作者: XIAOYUEHAN 时间: 2009-12-23 06:55

b当中 within the size就是充分条件吧，ls？

作者: Avantasia 时间: 2009-12-23 20:29

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。

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