By the end of 2002, the public transport of Delhi switched over to a new fuel. Name the fuel. Why is this fuel considered better?

Delhi is the fourth most polluted city in the world in a list of 41 cities. The burning of fossil fuels like petrol and diesel has added to the pollution of air in Delhi. So, in the year 2002 it switched over to a new fuel - CNG (compressed natural gas). It is considered better because CNG is a clean fuel that produces very little unburnt particles and thus it is considered as eco-friendly. CNG burns most efficiently, unlike petrol or diesel, in the automobiles and very little of it is left unburnt. Moreover, CNG is cheaper than petrol or diesel, cannot be siphoned off by thieves and adulterated like petrol or diesel.