Flux is used to remove

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

Carbon cannot reduce $Fe_2O_3$ to $Fe$ at a temperature below $983 \ K$ because

The slag consists of molten impurities generally in the form of

Copper is extracted from sulphide ore using the method:

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In the Ellingham diagram,a sharp change in slope of the line is observed for $Mg \rightarrow MgO$ at $\sim 1120^{\circ} C$.
Reason $R$: There is a large change of entropy associated with the change of state.
In the light of the above statements,choose the correct answer from the options given below.