In the leaching of `Ag_(2)S` with `NaCN`, a stream air is also passed. It is because of
A. Reversible nature of reaction between `Ag_(2)S` and `NaCN`
B. Oxidised `Na_(2)S` formed into `Na_(2)SO_(4)`
C. Irreversible nature of reaction between `Ag_(2)S` and `NaCN`
D. None of above