Following solutions were prepared by mixing different volumes of NaOH and HCl of different concentrations:

a. 60 mL 10 M HCl + 40 mL 10 M NaOH
b. 55 mL 10 M HCl + 45 mL 10 M NaOH
c. 75 mL 5 M HCl + 25 mL 5 M NaOH
d. 100 mL 10 M HCl + 100 mL 10 M NaOH