Hydrocarbon emissions from vehicles are due to partial burning of fuel and contribute to the air pollution. To reduce the amount of hydrocarbon emissions (and other pollutants), the use of CNG (Compressed Natural Gas) as an alternative to petrol and diesel is being tried in some Indian cities.
The hydrocarbon emissions for different vehicles using petrol/diesel and CNG as fuel are shown in the graph below.



For which of these vehicles is the percentage reduction in the amount of hydrocarbon emission the most?