Cyanide ion is a very good complexing agent and also functions as a reducing agent. Addition of an aqueous solution of $$\mathrm{KCN}$$ to a solution of copper sulfate yields a white precipitate which is soluble in excess aqueous $$\mathrm{KCN}$$ to form the complex

A

$$[\mathrm{Cu(CN)_4}]^-$$

B

$$[\mathrm{Cu(CN)_4}]^{2-}$$

C

$$[\mathrm{Cu(CN)_4}]^{3-}$$

D

$$[\mathrm{Cu(CN)_4}]^{4-}$$