The logic gates in which all the inputs must be low to get a high output are

Truth table for AND, OR, NAND and NOR gate for two input $A$ and $B$ are given below. AND gate \begin{array}{|ccc|} \hlineA & B & Output Y=A B \\ \hline 0 & 0 & 0 \\ \hline 0 & 1 & 0 \\ \hline 1 & 0 & 0 \\ \hline 1 & 1 & 1 \\ \hline \end{array} OR gate \begin{array}{|llc|} \hlineA & B & Output Y=A+B \\ \hline 0 & 0 & 0 \\ \hline 0 & 1 & 1 \\ \hline 1 & 0 & 1 \\ \hline 1 & 1 & 1 \\ \hline \end{array} NOR gate \begin{array}{|cccc|} \hlineA & B & A+B & Output Y=\bar{A}+\bar{B} \\ \hline 0 & 0 & 0 & 1 \\ \hline 0 & 1 & 1 & 0 \\ \hline 1 & 0 & 1 & 0 \\ \hline 1 & 1 & 0 & 0 \\ \hline \end{array} NAND gate \begin{array}{|rrrc|} \hlineA & B & A+B & Output Y=\overline{A B} \\ \hline 0 & 0 & 0 & 1 \\ \hline 0 & 1 & 0 & 1 \\ \hline 1 & 0 & 0 & 1 \\ \hline 1 & 1 & 1 & 0 \\ \hline \end{array} From above truth tables, it is clear that in NOR and NAND gate, if all the inputs are low, then their outputs are high.