In the following reaction sequence, the correct structure($s$) of $X$ is (are)

$A$ body is executing simple harmonic motion. At a displacement $x$ its potential energy is $E_1$ and at a displacement $y$ its potential energy is $E_2$. The potential energy $(E)$ at displacement $(x+y)$ is

In the hardening stage of plaster of Paris,the compound formed is:

The gas that cannot be collected over water is

If $A$ and $B$ are square matrices of order $3$,$A$ is non-singular,and $AB = O$,then $B$ is a:

Which of the following equations represents a circle?