Chloramine-$T$ is a

$A$ disc of radius $R$ is made to oscillate about a horizontal axis passing through its periphery. Its time period would be

Which of the following statements is correct?

Identify the reagent used in the following reaction.
Benzoic acid $\stackrel{\text{Reagent}}{\Delta}$ Benzoyl chloride + Phosphorous oxychloride + Hydrogen chloride

The relation "congruence modulo $m$" is

If $u=\sin ^{-1}\left(\frac{x^4+y^4}{x+y}\right)$,then $x \frac{\partial u}{\partial x}+y \frac{\partial u}{\partial y}$ is equal to