In the extraction of silver by Mac-Arther cyanide process, a small amount of KNO3 is also added as a flux. The function of KNO3 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 SO2 which escapes from the reaction