You are asked to make a buffer with a pH value close to 4 that would best resist an increase in pH. You can select one of the following acid–conjugate base pairs: acetic acid–acetate, K_a = 1.8 x 10^-5; ammonium ion–ammonia, K_a = 5.6 x 10^-10; or benzoic acid–benzoate, K_a = 6.3 x 10^-5; and you can mix them in the following acid–conjugate base ratios: 1:1, 2:1, or 1:2. What combination would make the best buffer?