Ammonia forms the complex ion $[Cu(NH_3)_4]^{2+}$ with copper ions in the alkaline solutions but not in acidic solutions. What is the reason for it ?

Question

( A ) In alkaline solutions insoluble $Cu(OH)_2$ is precipitated which is soluble in excess of any alkali
( B ) Copper hydroxide is an amphoteric substance
( C ) In acidic solutions hydration protects copper ions
( D ) In acidic solutions protons co-ordinate with ammonia molecules forming $NH_4^+$ ions and $NH_3$ molecules are not available