When $C_6H_5NH_3^+ Cl^-$ salt is dissolved in water,it undergoes hydrolysis. Find the equilibrium constant of the hydrolysis reaction if the ionization constant of the weak base is $10^{-4}$.

• A
  $10^{-4}$
• B
  $2 \times 10^{-4}$
• C
  $10^{-10}$
• D
  $2 \times 10^{-10}$