Which of the reaction is suitable for concentrating ore by leaching process ?

1. A $$ 2 \mathrm{Cu}_{2} \mathrm{~S}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Cu}_{2} \mathrm{O}+2 \mathrm{SO}_{2} $$
2. B $$\mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{CO} \rightarrow 3 \mathrm{FeO}+\mathrm{CO}_{2}$$
3. C $$\mathrm{Al}_{2} \mathrm{O}_{3}+2 \mathrm{NaOH}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right]$$ Correct answer
4. D $$\mathrm{Al}_{2} \mathrm{O}_{3}+6 \mathrm{Mg} \rightarrow 6 \mathrm{MgO}+4 \mathrm{Al}$$