Which of the following is not a biofuel?

• ethanol
  Ethanol and methanol are biofuels created when carbohydrates from various plants are distilled into alcohol. Natural gas is a fossil fuel.
• natural gas
  Ethanol and methanol are biofuels created when carbohydrates from various plants are distilled into alcohol. Natural gas is a fossil fuel.
• methanol
  Ethanol and methanol are biofuels created when carbohydrates from various plants are distilled into alcohol. Natural gas is a fossil fuel.
• butanol
  Ethanol and methanol are biofuels created when carbohydrates from various plants are distilled into alcohol. Natural gas is a fossil fuel.