Transportation has been made much easier thanks to the invention of cars. However, cars are the greatest contributor of air pollution.

A The invention of cars has made transportation much easier, but cars are among the greatest contributors air pollution.     

B  Although the invention of cars has made transportation much easier, people use the cars to contribute to the pollution of air.

C Although the invention of cars has made transportation much easier, cars are the greatest contributor of air pollution.

D However easier the invention of cars has made transportation, it is cars that are among the greatest contributors of air pollution.