Transportation has been made much easier thanks to the invention of car. 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 of air pollution.
• B. However easier the invention of cars has made transportation, it is cars that among the greatest contributors of air pollution.
• C. Although the invention of cars has made transportation much easier, cars are the greatest contributor of air pollution of air.
• D. Although the invention of cars has made transportation much easier, people use cars to contribute to the pollution of air.