The solubility product of a salt having general formula ${\mathrm{MX}}_2$ in water is $4 \times {10}^{-12}.$  The concentration of ${\mathrm{M}}^{2+}$ ions in the aqueous solution of the salt is

• A

  $2 \times {10}^{-6} \mathrm{M}$

• B

  $1 \times {10}^{-4} \mathrm{M}$

• C

  $1.6 \times {10}^{-4} \mathrm{M}$

• D

  $4.0 \times {10}^{-6} \mathrm{M}$