An aqueous solution contains 0.10 M H₂S and 0.20 M HCl. If the equilibrium constants for the formation HS− from H₂S is 1.0×10⁻⁷ and that of S₂− from ions HS− is 1.2×10⁻¹³ then the concentration of S₂− ions in aqueous solution is:
A. 3×10⁻²⁰
B. 6×10⁻²¹
C. 5×10⁻¹⁹
D. 5×10⁻⁸