For the diprotic weak acid H₂A, Ka1 = 2.1 x 10⁻⁶ and Ka₂ = 7.9 x 10⁻⁹. What is the pH of a 0.0400 M solution of H₂A?
a) 1.23
b) 2.34
c) 3.45
d) 4.56