In the Following Reaction,which Species Is the Lewis Acid? Cu²⁺(aq) $\leftrightarrows$

In the following reaction,which species is the Lewis acid? Cu²⁺(aq) + NH₃(aq) $\leftrightarrows$ Cu(NH₃) ²⁺(aq)

A) Cu²⁺
B) NH₃
C) [Cu(NH₃) ]²⁺
D) None of these is an acid.
E) All of these are acids.