[求助]关于A-->B, LAST section1 No. 6

fengyun1

6. No senator spoke at the convention unless he or she was Democrat. No Democrat both spoe at the convention and was a senator.

Which one of the following conclusions can be correctly drawn from the statements above?
A. No one but senators spoke at the convention.
B. No Democrat spoke at the convention.
C. Only Democrats spoke at the convention.
D.No senator spoke at the convention.
E. Some Democrat senators spoke at the convention.

Answer is D.
本来认为A-->B很容易,但这题让我考虑了很久,不知该如何下手,求教各位!先谢过!!!

dorbear

A: Senator; B: Spoke; C: Democrat

A&B => C
(No senator spoke at the convention unless he or she was Democrat)

A&B => -C, C=> ~(A&B)
(No Democrat both spoke at the convention and was a senator)

A&B => C => ~(A&B)
So, A&B => ~(A&B),  A&B = {0}

The two Elements: "Senator" and "Spoke" cannot Co-exit.

fengyun1

有点意思,多谢!
我就是在A-->-A之后,手足无措!差距呀!

风云