What is the incorrect reason for the statement "In many reactions, hydrogen acts as a reducing agent but it is not widely used as a reducing agent for metal extraction"?

Explain the role of the following in the processes mentioned :

Limestone in the metallurgy of iron.

Explain the following:

Flux

Identify the auto reduction reactions from the following:

$\mathrm{a})$ ${\mathrm{Fe}}_3{\mathrm{O}}_4+4\mathrm{CO}⟶3\mathrm{Fe}+4{\mathrm{CO}}_2$

$\mathrm{b})$ ${\mathrm{Cu}}_2\mathrm{O}+\frac{1}{2}{\mathrm{Cu}}_2\mathrm{S}⟶3\mathrm{Cu}+\frac{1}{2}{\mathrm{SO}}_2$

$\mathrm{c})$ $\mathrm{PbS}+2\mathrm{PbO}⟶3\mathrm{Pb}+{\mathrm{SO}}_2$

$\mathrm{d})$ ${\mathrm{Cu}}_2\mathrm{O}+\mathrm{C}⟶2\mathrm{Cu}+\mathrm{CO}$