The NAND gate won't work if ______.

The NAND gate operates by producing a low output only when both of its inputs are high (on). In every other combination of inputs, the output of the NAND gate will be high (on). Therefore, when both inputs are powered on, or high, the NAND gate cannot function correctly since it is designed to produce a low output in this specific scenario. The nature of the NAND gate is such that it effectively negates the AND operation; it combines the role of logic AND with a NOT function. Hence, the output is high unless it receives the condition of both inputs being high.

In contrast, the other scenarios (where one input is on, both inputs are off, or one input is off) do not lead to the NAND gate failing to function; instead, these cases will yield a high output, demonstrating that the gate is indeed working as intended.