Which one of the following is a reduction reaction?

• A. 2Mg(s) + O2(g) → 2MgO(s)
• B. S(s) + O2(g) → SO2(g)
• C. \(2HgO\left( s \right)\overset{heat}{\mathop{\to }}\,2Hg\left( l \right)+{{O}_{2}}\left( g \right)\)
• D. Mg(s) + S(s) → MgS(s)