$K_a$ for HCN is $5 \times 10^{-10}$ at $25^\circ C$. How much volume of 5 M KCN solution is required to be added to 10 mL of 2 M HCN in order to maintain $pH$ = 9

Question

(a) 2 mL

(b) 5 mL

(c) 3 mL

(d) 4 mL