跪求牛人帮忙解答逻辑谜题
题目如下：


A, B and C are either telling the truth or they are lying (in which case what they say is false.)





A is telling the truth when he says: B and C are both lying.


B is telling the truth when she says: C is lying.


C is telling the truth when he says: A and B are both telling the truth.


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Therefore, B is telling the truth and C is lying.



Invalid.

Sound.

Valid but unsound.

Valid but soundness cannot be determined.