31. Which of the following solutions will be the best buffer at a pH of 4.74? (Ka for acetic acid, HC2H3O2, is 1.8 × 10-5, Ka for benzoic acid, HC7H5O2, is 6.4×10-5) (A) 0.10 M HC2H3O2 + 0.10 M NaC2H3O2 (B) 5.0 M HC2H3O2 + 5.0 M Na C2H3O2 (C) 0.50 M HC7H5O2 + 0.14 M KC7H5O2 (D) 5.0 M HC7H5O2 + 1.4 M KC7H5O2