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

1. A
   $2.0\times 10^{-6}\text{M}$
2. B
   $1.0\times 10^{-4}\text{M}$
3. C
   $1.6\times 10^{-4}\text{M}$
4. D
   $4.0\times 10^{-10}\text{M}$