Match Column $-I$ with Column $-II$ and select the correct answer using the codes given below

Chromium is obtained by reducing concentrated chromite ore with

$20\,mL$ of $0.5\,M\,NaCl$ is required to coagulate $200\,mL$ of $As _2 S _3$ solution in $2$ hours. The coagulating value of $NaCl$ is $………..$.

Dow's process

Formation of metallic copper from the sulphide ore in the normal thermo-metallurgical process essentially involves which one of the following reaction.