Ammonia forms the complex ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}$ with copper ions in alkaline solution but not in acid solution. The reason for it is:

• A

  in alkaline solution Cu(OH)₂ is precipitated which is soluble in excess of alkali

• B

  in acidic solution protons coordinates with ammonia molecule forming $N{H}_4^{+}$ ions and NH₃ molecules are not available.

• C

  copper hydroxide is amphoteric

• D

  in acidic solution hydration protects Cu²⁺ ions