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

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A. In acidic solution, the protons co -ordinate with the ammonia molecule forming `NH_4^+` ion and `NH_3` molecules are not available.
B. In alkaline solution, insoluble `Cu(OH)_2` is precipitated which is soluble in any alkali
C. `Cu (OH)_3` isan amphoteric substance
D. In acidic solution, hydration protects copper ion.