c - CNG
CNG (Compressed Natural Gas) - A greener way to power a car, producing less carbon dioxide emissions than a petrol engine. Most CNG-fuelled cars use both petrol and CNG, although the two fuels are never mixed. The driver switches between the two as required.
The downside is that CNG takes up four times the space of petrol to provide the same amount of energy. Few filling stations sell CNG, and converting a car to run on CNG is expensive. (See also Alternative fuels.)