A thin disc of radius $b=2a$ has a concentric hole of radius $'a'$ in it (see figure). It carries uniform surface charge $'\sigma'$ 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 : <br>

Question

( A ) $\frac{\sigma}{4 a \varepsilon_0}$
( B ) $\frac{\sigma}{ a \varepsilon_0}$
( C ) $\frac{\sigma}{8 a \varepsilon_0}$
( D ) $\frac{\sigma}{2 a \varepsilon_0}$