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Air pollution has steadily decreased since 1999, before suddenly increasing is the statement best describes air quality in Houston since 1999. As the use of mass transit and transitioning to fuel-efficient or low-emission vehicles.
What is low-emission?
A car with low emissions produces very little pollution. A low-emission zone is an area where polluting cars are either not allowed or must pay to enter. They've developed a new kind of low-emission buses.
Vehicles that are either designated as Zero emission Vehicles or as Low Emission Vehicles are considered low-emitting and fuel efficient.
Thus, option D is correct.
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