28. 以下何者為路易士酸?
(A) \(\mathrm{BF}_4^{-}\)(B) \(\mathrm{C}_{5} \mathrm{H}_{7} \mathrm{~N}\) (吡啶)(C) \(\mathrm{Cu}^{2+}\)(D) \(\mathrm{Mg}^{2+}\)(E) \(\mathrm{PH}_3\)