In the extraction of iron, which of the following reactions pertains to the formation of slag in the blast furnace?

1. A
   $\text{CaO(s)}+{\text{SiO}}_2\text{(s)}\to {\text{CaSiO}}_3\text{(s)}$
2. B
   $2\text{C(s)}+{\text{O}}_2\text{(g)}\to 2\text{CO(g)}$
3. C
   ${\text{Fe}}_2{\text{O}}_3\text{(s)}+3\text{CO(g)}\to 2\text{Fe(l)}+3{\text{CO}}_2\text{(g)}$
4. D
   ${\text{CaCO}}_3\text{(s)}\to \text{CaO(s)}+{\text{CO}}_2\text{(g)}$