令＃为定义为X＃Y = X’+ Y’的二进制运算符，其中X和Y是布尔变量。考虑以下两个语句。
S1: (P # Q) # R = P # (Q # R)
S2: Q # R = R # Q 对于布尔变量P，Q和R，以下哪一项是正确的?
(A)只有S1为真
(B)只有S2为真
(C) S1和S2均为True
(D) S1和S2都不为真答案： (B)
解释：
S2 is true, as X' + Y' = Y' + X'
S1 is false.
Let P = 1, Q = 1, R = 0, we get different results
(P # Q) # R = (P' + Q')' + R' = (0 + 0)' + 1 = 1 + 1 = 1
P # (Q # R) = P' + (Q' + R')' = 0 + (0 + 1)' = 0 + 0 = 0
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