In the context of atmospheric chemistry and sustainable biofuel development, identifying the structural classification of organic compounds is essential. Which of the following substances is classified as an unsaturated hydrocarbon?
C8H18\text{C}_8\text{H}_{18}C8H18
C5H8\text{C}_5\text{H}_8C5H8
C18H34O2\text{C}_{18}\text{H}_{34}\text{O}_2C18H34O2
C2H6O\text{C}_2\text{H}_6\text{O}C2H6O