A thin disc of radius $b=2a$ has a concentric hole of radius ‘a’ in it (see figure). It carries uniform surface charge ‘s’ on it. If the electric field on its axis at height $‘h’ (h<<a)$ from its centre is given as ‘'Ch'’ then value of ‘C’ is :

Question

$\begin{array}{1 1} \frac{\sigma} {a\in_0} \\ \frac{\sigma} {2a\in_0} \\ \frac{\sigma} {4a\in_0} \\ \frac{\sigma} {8a\in_0} \end{array}$