Citric acid is a triprotic acid. Its acid dissociation constants at 25°C are pKa1-pKa3: 3.13, 4.76, and 6.4 Which one of the following is the approximate pH at 25°C of a solution formed by mixing equal volumes of equimar (0.2 M) solutions of citric acid and potassium hydroxide?
A. 8.5
B. 5.1
C. 6.4
D. 3.9