Z^- Is a Weak Base

Z^- is a weak base. An aqueous solution of NaZ is prepared by dissolving 0.350 mol of NaZ in sufficient water to yield 1.0 L of solution. The pH of the solution was 8.93 at 25.0 °C. The K_b of Z^- is ________.

A) 1.2 × 10^-5
B) 6.9 × 10^-9
C) 2.1 × 10^-10
D) 9.9 × 10^-2
E) 2.8 × 10^-12