A $N-P-N$ transistor conducts when, 

The logic circuit shown below has the input waveforms $\mathrm{A}$ and $\mathrm{B}$, as shown.

Pick out the correct output waveform.

The arrangement of $\mathrm{NAND}$ gates shown below effectively works as

The combinations of $\mathrm{NAND}$ gates shown here are equivalent to:

For the given logic gate circuit, to get output as $1$ at $R,$ find the input of $X,$ if input value of $Y$ is zero.