A 200-ML Sample of 0

A 20.0-mL sample of 0.50 M H₂C₆H₆O₆ (ascorbic acid, a diprotic acid) was titrated with 0.50 M NaOH. The following data were gathered during the titration. What is K_a2 for ascorbic acid?

A) 6.8 *10¯⁵
B) 6.2 * 10¯⁶
C) 6.2 * 10¯⁷
D) 6.2 * 10¯⁸
E) 2.8 * 10¯¹²