Which one of the following is a balanced equation?

A. Na (s) + MgF2 → 2NaF (s) + Mg (s)
B. 2Na (s) + MgF2 → 2NaF (s) + Mg (s)
C. Na (s) + O2 (g) → Na2O
D. Zn (s) + HCl (g) → ZnCl2 (s) + H2 (g)