$138 \ g$ of ethyl alcohol is mixed with $72 \ g$ of water. The ratio of mole fraction of alcohol to water is

$A$ plane mirror is placed at the bottom of a tank containing a liquid of refractive index $\mu$. $P$ is a small object at a height $h$ above the mirror. An observer $O$,vertically above $P$ outside the liquid,sees $P$ and its image in the mirror. The apparent distance between these two will be

The line among the following which touches the parabola $y^2=4ax$ is

How many cyclic structures are possible for $C_4H_6$?

Which of the following chlorides is covalent and soluble in ether?

For a perfectly black body,the coefficient of emission (emissivity) is: