Which means of transportation does NOT cause air pollution: car, bike, bus, truck？
A means of doing something is a method, instrument, or process which can be used to do it. Means is both the singular and the plural form for this use. 方法; 手段; 过程
Transportation refers to any type of vehicle that you can travel in or carry goods in. 交通工具
To cause something, usually something bad, means to make it happen. 引起
Pollution is the process of polluting water, air, or land, especially with poisonous chemicals. 污染
Pollution caused by transportation
Bicycling significantly reduces transportation emissions while also reducing traffic congestion and the need for petroleum. The total number of pounds of pollutants, (comprised of hydrocarbons, carbon monoxide, nitrogen oxides and carbon dioxide), emitted per year is 12,140.30 lbs/year (or 0.97 lbs/mile) for passenger cars and 17,025.80 lbs/year (or 1.21 lbs/mile) for light trucks.