Astronomers detect four molecules in the atmosphere of a distant planet. Their formulae are C3H8\text{C}_3\text{H}_8C3H8, C2H4\text{C}_2\text{H}_4C2H4, C5H12\text{C}_5\text{H}_{12}C5H12 and C2H6O\text{C}_2\text{H}_6\text{O}C2H6O.
Which of these molecules is not a hydrocarbon?
C2H4\text{C}_2\text{H}_4C2H4
C5H12\text{C}_5\text{H}_{12}C5H12
C2H6O\text{C}_2\text{H}_6\text{O}C2H6O
C3H8\text{C}_3\text{H}_8C3H8