The addition of small amounts of either acid or base to a buffer solution causes only small changes in $pH$ because the buffer solution

• A
  Does not contain either $H_3O^{+}$ or $OH^{-}$
• B
  Contains large amounts of both $H_3O^{+}$ and $OH^{-}$
• C
  Reacts with the added acid or base
• D
  Contains a strong acid and the salt of the same strong acid