Nitric Acid, HNO₃, Is Produced Along with Water by the Reaction

Nitric acid, HNO₃, is produced along with water by the reaction of oxygen, O₂, with ammonia, NH₃, and water, H₂O, in a series of steps, with the unbalanced net reaction shown below.
__ NH₃ + __ O₂ $\rarr$ __ HNO₃ + __H₂O
If the coefficient of NH₃ is 1 in the balanced chemical equation of the reaction, what is the coefficient on O₂?

A) 2
B) 3
C) 4
D) 5
E) 6