The value of $K^{sp}$ of $HgCl_2$ at room temperature is $4 \times 10^{-15}$. The concentration of $Cl^-$ ion in its aqueous solution at saturation point is

Question

(a) $1 \times 10^{-5}$

(b) $2 \times 10^{-5}$

(c) $2 \times 10^{-15}$

(d) $8 \times 10^{-15}$