The Complete Combustion of Propane,C₃H₈,yields Carbon Dioxide and Water

The complete combustion of propane,C₃H₈,yields carbon dioxide and water:
_- C₃ H₈ + _- O₂→ _- CO_{2 + -} H₂O
The smallest whole-number coefficient of oxygen in the balanced equation is

A) 6.
B) 3.
C) 7.
D) 4.
E) 5.