An aqueous solution contains  $0.10 \mathrm{M}\mathrm{ }{\mathrm{H}}_2\mathrm{S}$ and $0.20 \mathrm{M} \mathrm{HCl}$. If the equilibrium constant for the formation of ${\mathrm{HS}}^{-}$ from ${\mathrm{H}}_2\mathrm{S}$ is $1.0\times {10}^{-7}$ and that of ${\mathrm{S}}^{2-}$ from ${\mathrm{HS}}^{-}$ ions is $1.2\times {10}^{-13}$, then, the concentration of ${\mathrm{S}}^{2-}$ ions in the aqueous solution is:

• A

  $5\times {10}^{-19}$
• B

  $5\times {10}^{-8}$
• Correct answer
  C

  $3\times {10}^{-20}$
• D

  $6\times {10}^{-21}$

Hence, the correct answer is option: 'C'.