Which of the following gives a buffer solution when equal volumes of the two solutions are mixed? (a) 0.10 M NH3 and 0.10 M HCl (b) 0.20 M NH3 and 0.10 M HCl (c) 0.10 M NH4Cl and 0.10 M NH3 (d) 0.20 M NH4Cl and 0.10 M NaOH