The study of the interaction of antigens and antibodies in the blood is known as:

$A$ particle is released from height $S$ from the surface of the Earth. At a certain height its kinetic energy is three times its potential energy. The height from the surface of the Earth and the speed of the particle at that instant are respectively:

Consider the two following statements $A$ and $B$,and identify the correct choice given in the answers: $(A)$ Duddell's thermo galvanometer is suitable to measure direct current only. $(B)$ Thermopile can measure temperature differences of the order of $10^{-3} {}^{\circ}C$.

An inductance of $1 \ H$ is connected in series with an $AC$ source of $220 \ V$ and $50 \ Hz$. The inductive reactance (in ohm) is (in $\pi$)

When a $p-n$ diode is reverse biased,then

The value of $\cot \left(\sum_{n=1}^{23} \cot ^{-1}\left(1+\sum_{k=1}^n 2 k\right)\right)$ is