Mexico City sits on an inland plateau  surrounded by volcanic peaks, which makes air quality a constant concern for people who live here. In April, the country took a decisive step toward improving air quality by enacting  a temporary ban  on private and federal vehicles in the city. The rule forbids people from driving in the city one day each week and one Saturday each month. Electric vehicles, government service vehicles, public transport options and school buses are not included in the ban. Mexico City isn't the first urban centre to be involved in car-free living. But the ban is more than an awareness-raiser; it was enacted with the direct aim of solving air pollution. In March, the city sank into a deep brown haze of smog when pollution levels passed the 200 mark. The city ordered some 1.1m of the area's 4.7m cars off the streets and also offered free bus and subway rides. Mexico City's temporary vehicle ban raises questions about the best ways to improve urban air quality. India, whose citizens breathe some of the world's dirtiest air, has tried a variety of solutions. Earlier this year, the Indian government started a 4% sales tax on new-car purchases. Beijing also has experience with cleaning the air. The city introduced alternate-day rules in advance of the 2008 Olympic Games, which produced good, if short-lived, results. The day after the vehicle-use restrictions   were _ , the blue faded back to gray. And then there is the London congestion  charge, introduced in 2003. The charge has had a measurable effect on air quality. For now, Mexico City--named by the United Nations as the most polluted city on the planet in 1992--can look forward to the start of the region's rainy season, when daily showers will help to clean the air. What makes the air quality of Mexico City a concern for citizens?
Its population. Its location. Its public transportation. Its government action.
Question: Mexico City sits on an inland plateau  surrounded by volcanic peaks, which makes air quality a constant concern for people who live here. In April, the country took a decisive step toward improving air quality by enacting  a temporary ban  on private and federal vehicles in the city. The rule forbids people from driving in the city one day each week and one Saturday each month. Electric vehicles, government service vehicles, public transport options and school buses are not included in the ban. Mexico City isn't the first urban centre to be involved in car-free living. But the ban is more than an awareness-raiser; it was enacted with the direct aim of solving air pollution. In March, the city sank into a deep brown haze of smog when pollution levels passed the 200 mark. The city ordered some 1.1m of the area's 4.7m cars off the streets and also offered free bus and subway rides. Mexico City's temporary vehicle ban raises questions about the best ways to improve urban air quality. India, whose citizens breathe some of the world's dirtiest air, has tried a variety of solutions. Earlier this year, the Indian government started a 4% sales tax on new-car purchases. Beijing also has experience with cleaning the air. The city introduced alternate-day rules in advance of the 2008 Olympic Games, which produced good, if short-lived, results. The day after the vehicle-use restrictions   were _ , the blue faded back to gray. And then there is the London congestion  charge, introduced in 2003. The charge has had a measurable effect on air quality. For now, Mexico City--named by the United Nations as the most polluted city on the planet in 1992--can look forward to the start of the region's rainy season, when daily showers will help to clean the air. What makes the air quality of Mexico City a concern for citizens?
A. Its population.
B. Its location.
C. Its public transportation.
D. Its government action.
Answer: