Which gate outputs true when the inputs differ?

• A. XOR gate
• B. AND gate
• C. NOR gate
• D. OR gate

Answer: (A)

The concept being tested is how a two-input gate behaves when the inputs are different. The gate that outputs true exactly in that situation is the exclusive OR: it gives a true output if one input is 1 and the other is 0, and a false output if both inputs are 0 or both are 1. For example, with inputs 0 and 1 (in either order) the output is true; with 0 and 0 or 1 and 1 the output is false.

This fits the idea of outputs true when inputs differ. In contrast, an AND gate only outputs true when both inputs are true; an OR gate outputs true if at least one input is true; a NOR gate outputs true only when both inputs are false.