If $a = \sqrt{21} - \sqrt{20}$ and $b = \sqrt{18} - \sqrt{17}$,then

Consider the following statements $A$ and $B$ and identify the correct answer given below:
$(A)$ $A$ body initially at rest is acted upon by a constant force. The rate of change of its kinetic energy varies linearly with time.
$(B)$ When a body is at rest,it must be in equilibrium.

The number of moles of electrons required to deposit $36 \ g$ of $Al$ from an aqueous solution of $Al(NO_3)_3$ is (At. wt. of $Al = 27$)

$A$ man standing on a road has to hold his umbrella at $30^{\circ}$ with the vertical to keep the rain away. He throws the umbrella and starts running at $10 ~km/h$. He finds that raindrops are hitting his head vertically. The actual speed of raindrops is:

In $\Delta ABC,$ if $\cos A + \cos C = 4\sin^2\frac{B}{2},$ then $a, b, c$ are in

The speed of sound in oxygen $(O_2)$ at a certain temperature is $460 \; m/s$. The speed of sound in helium $(He)$ at the same temperature will be (assume both gases to be ideal): (in $; m/s$)