If the $pK_b$ of the fluoride ion $(F^-)$ at $25^{\circ}C$ is $10$,what is the ionization constant $(K_a)$ of hydrofluoric acid $(HF)$ at the same temperature?

• A
  $1 \times 10^{-5}$
• B
  $1 \times 10^{-3}$
• C
  $1 \times 10^{-4}$
• D
  $1 \times 10^{-2}$