A solution of weak acid was titrated with base NaOH. The equivalence point was reached when 36.12 mL of 0.1M NaOH have been added. Now, 18.06 mL of 0.1M HCl were added to titrated solution, the pH was found to be 4.92. Ka of acid is
A Ka=1.0×10⁻⁵
B. Ka=1.3×10⁻⁵
C. Ka=1.2×10⁻⁵
D. Ka=1.1×10⁻⁵