Which weak acid would be best to use when preparing a buffer solution with a pH of 9.40? a. an acid with pKa = 4.20 b. an acid with pKa = 10.60 c. an acid with pKa = 5.30 d. an acid with pKa = 7.20 e. an acid with pKa = 9.38 f. an acid with pKa = 6.10