The solubility product of a sparingly soluble salt $AX_2$ is $3.2\times 10^{-11}$. Its solubility (in moles/litre) is:-

Question

( A ) $3.1\times 10^{-4}$
( B ) $4\times 10^{-4}$
( C ) $5.6\times 10^{-6}$
( D ) $2\times 10^{-4}$