The resultant gate and its Boolean expression in the given circuit is

When a small amount of impurity atoms are added to a semiconductor then generally its resistivity

In the logic circuit diagram, when all the four inputs, A, B, C, D are 'one' the outputs $Y_1$, $Y_2$, $Y_3$ are respectively (1, 1, 0). When the inputs A and C are changed to zero and B and D are still 'one', then the outputs $Y_1$, $Y_2$, $Y_3$ are respectively change to

In the diagram shown, the resistance between points A and B is '$R_1$' when an ideal diode D is forward biased and is '$R_2$' when ideal diode D is reverse biased. The ratio $R_1 / R_2$ is