During which reaction is dihydrogen gas not produced under normal conditions?

$(a)$ Carbon cannot be used as a reducing agent to obtain $Mg$ from $MgO$. Why?
$(b)$ How is sodium obtained from molten sodium chloride? Give the equations of the reactions.
$(c)$ How is copper obtained from its sulphide ore? Give the equations of the reactions.

Name two metals in each of the following cases:
$(i)$ Metals that are stored in kerosene.
$(ii)$ Metals that are alloyed with iron to make stainless steel.
$(iii)$ Metals which are highly malleable and ductile.

In which solvent is sulphur soluble?

If copper is kept open in air,it slowly loses its shining brown surface and gains a green coating. It is due to the formation of

A non-metal $A$, which is the largest constituent of air, when heated with $H_2$ in a $1 : 3$ ratio in the presence of a catalyst $(Fe)$, gives a gas $B$. On heating with $O_2$, it gives an oxide $C$. If this oxide is passed into water in the presence of air, it gives an acid $D$, which acts as a strong oxidising agent.
$(a)$ Identify $A$, $B$, $C$, and $D$.
$(b)$ To which group of the periodic table does this non-metal belong?