Sorbinic acid, H₂Srb, is diprotic with pKₐ₁ = 4.2 and pKₐ₂ = 8.1. Which pair of solutes, present in similar concentrations, would create the most effective buffer with pH close to 8.1?
a) H₂Srb and NaHSrb
b) NaHSrb and Na₂Srb
c) HCl and NaCl
d) H₂Srb and Na₂Srb