$$A,B,C$$ and $$D$$ are input bits, and $$Y$$ is the output bit in the $$XOR$$ gate circuit of the figure below. Which of the following statements about the sum $$S$$ of $$A,B,C,D$$ and $$Y$$ is correct?

In the figure, as long as $${X_1} = 1$$ and $${X_2} = 1,$$ the output $$Q$$ remains

A digital circuit which compares two numbers $${A_3}{A_2}{A_1}{A_0},\,\,{B_3}{B_2}{B_1}{B_0}$$ is shown in Fig. To get output $$Y=0,$$ choose one pair of correct input numbers

For the circuit shown in Fig. the Boolean expression for the output $$Y$$ in terms of inputs $$P,$$ $$Q,$$ $$R$$ and $$S$$ is