Which of the following has the lowest boiling point?

$\left(\frac{1+i}{1-i}\right)^4+\left(\frac{1-i}{1+i}\right)^4$ is equal to

If the lines,joining the origin to the points of intersection of the curve $2x^2 - 2xy + 3y^2 + 2x - y - 1 = 0$ and the line $x + 2y = k$,are at right angles,then $k^2$ equals

The temperature of $5$ moles of a gas at constant volume is changed from $100^{\circ} C$ to $120^{\circ} C$. The change in internal energy is $80 \ J$. The total heat capacity of the gas at constant volume in $J/K$ is:

Which one is an incorrect statement?

The method of zone refining of metals is based on the principle of: