Which balances the equation Mg plus cap o sub 2 rightarrow MgO ?

Oxygen is diatomic, so the reactant has 2 oxygen atoms in O₂. Put a 2 before MgO to make 2 oxygen atoms on the product side; this also gives 2 magnesium atoms there, so put a 2 before Mg. The balanced equation is 2 Mg + O₂ → 2 MgO, which is choice C.

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