Ammonia forms the complex ion `[Cu(NH_3)_4]^(2+)` with copper ions in alkaline solutions but not in acidic solution. The reason for this is

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