In which of the following reactions does the hybridization of the central atom change from $sp^2$ to $sp^3$?

• A
  $NH_3 + H^{+} \to NH_4^{+}$
• B
  $AlF_3 + 3F^{-} \to AlF_6^{3-}$
• C
  $BF_3 + F^{-} \to BF_4^{-}$
• D
  $H_2O + H^{+} \to H_3O^{+}$