In the extraction of silver by the Mac-Arthur-Forrest cyanide process,a small amount of $KNO_3$ is also added as a flux. The function of $KNO_3$ is

• A
  to oxidise $Ag$ in the native form to $Ag^{+}$
• B
  to oxidise lead and zinc impurities
• C
  to form a complex with $Ag^{+}$ which is then reduced to metallic silver by using zinc
• D
  to oxidise the sulphur in the argentite ore to $SO_2$ which escapes from the reaction