I'm not sure if the answer is A or B... someone help

Which of the following equations has correct formulas and also is a balanced equation?

A.
Mg + O2 --> MgO2

B.
2Be + O2 --> 2BeO

C.
CaCO3 --> CaO + CO3

D.
3KCl + HNO3 --> 3HCl + KNO3