How does ammonia react with a solution of $Cu^{2+}$?

• A
  It forms a white precipitate of $Cu(OH)_2$.
• B
  It forms a deep blue complex $[Cu(NH_3)_4]^{2+}$.
• C
  It reduces $Cu^{2+}$ to $Cu^+$.
• D
  It forms a brown precipitate of $Cu_2O$.