Which of the following is a conjugate acid-base pair?

$A$ light wave enters from medium $1$ to medium $2$. Its velocity in the $2^{\text{nd}}$ medium is double that in the $1^{\text{st}}$ medium. For total internal reflection,the angle of incidence must be greater than ....... $^{\circ}$.

The chemicals and the reaction conditions required for the preparation of ethane are:

The figure shows a system of logic gates. From the study of the truth table,it can be found that to produce a high output $(1)$ at $R$,we must have:

Cornea transplant in humans is almost never rejected. This is because

For $|x| < 1$,the constant term in the expansion of $\frac{1}{(x-1)^2(x-2)}$ is