Following solutions were prepared by mixing different volumes of NaOH and HCl of different concentrations:$\\$a. $60$ mL $\frac{M}{10}$ HCl $+ 40$ mL $\frac{M}{10}$ NaOH$\\$b. $55$ mL $\frac{M}{10}$ HCl $+ 45$ mL $\frac{M}{10}$ NaOH$\\$c. $75$ mL $\frac{M}{5}$ HCl $+ 25$ mL $\frac{M}{5}$ NaOH$\\$d. $100$ mL $\frac{M}{10}$ HCl $+ 100$ mL $\frac{M}{10}$ NaOH$\\$pH of which one of them will be equal to 1?