Which gate outputs high when at least one input is false?

Which gate outputs high when at least one input is false?

• A. And Gate
• B. Or Gate
• C. Not Gate
• D. Nand Gate

Answer: (D)

Think of the gate as the negation of AND. An AND gate only goes high when every input is true. So if any input is false, the AND output would be low. The NAND gate flips that result, producing a high output whenever not all inputs are true—that is, whenever at least one input is false. If all inputs are true, the NAND output goes low. That’s why this gate is the best match for the condition “outputs high when at least one input is false.” The other gates don’t fit that rule consistently: AND would stay low if any input is false; OR would sometimes be high when there are true inputs and not guaranteed to be high just because one input is false; NOT only handles a single input and inverts it.

Think of the gate as the negation of AND. An AND gate only goes high when every input is true. So if any input is false, the AND output would be low. The NAND gate flips that result, producing a high output whenever not all inputs are true—that is, whenever at least one input is false. If all inputs are true, the NAND output goes low.

That’s why this gate is the best match for the condition “outputs high when at least one input is false.” The other gates don’t fit that rule consistently: AND would stay low if any input is false; OR would sometimes be high when there are true inputs and not guaranteed to be high just because one input is false; NOT only handles a single input and inverts it.