If $\frac{d^2y}{dx^2} = 0$,then

Which of the following exhibits moderate electrical conductivity?

The force between the plates of a parallel plate capacitor of capacitance $C$ and distance of separation of the plates $d$ with a potential difference $V$ between the plates,is

$\int \tan ^{-1}\left(\sqrt{\frac{1-x}{1+x}}\right) d x$ is equal to

The two lenses of an achromatic doublet should have:

The number of radial nodes of $3s$ and $2p$ orbitals respectively are