Which one of the following is a balanced equation?

A. 2Ca (s) + O₂ (g) → 2CaO (s)
B. 6Ca (s) + 2O₂ (g) → 6CaO (s)
C. 4Ca (s) + O₂ (g) → 2CaO (s)
D. 6Ca (s) + 4O₂ (g) → 2CaO (s)