Which of the following will not produce a buffered solution?

a) 100 ml of 0.1 M NaCO and 50 ml of 0.1 M HCl
b) 100 ml of 0.1 M NaHCO₃ and 25 ml of 0.2 M HCl
c) 50 ml of 0.2 M Na₂CO₃ and 5 ml of 1.0 M NaOH
d) 100 ml of 0.1 M Na₂CO₃ and 50 ml of 0.1 M NaOH