Which one of the following is NOT a valid identity?

• A. (x ⊕ y) ⊕ z = x ⊕ (y ⊕ z)
• B. (x + y) ⊕ z = x ⊕ (y + z)
• C. x ⊕ y = x + y, if xy = 0
• D. x ⊕ y = (xy + x'y')'