Which one of the following is not a balanced equation? Explain why.

A.C3H8 (g) + O2 (g) → CO2 (g) + H2O (g)

B.C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)

C.2Mg (s) + O2 (g) → 2MgO (s)

D.4Na (s) + O2 (g) → 2Na2O (s)