Scientists measured the impact that people have on the environment using a term called car

bon " footprint." That footprint reflects the amount of carbon dioxide that is emitted (排放)into the atmosphere as a result of someone s daily activities. Carbon footprints tend to be low for city dwellers(城镇居民). Living in a suburb outside a city, however, can turn that footprint into a bootprint. Energy researchers Christopher Jones and Daniel Kammen calculated carbon footprints for people in every zip code across the United States. People living in city centers had small footprints, the researchers found. "It is much easier to have a low carbon impact if your home is close to where you work, shop and play," explains Jones. Living within walking or biking distance cuts back on the amount of carbon dioxide associated with moving people by cars. And cities with extensive bus and subway networks allow people to travel great distances while keeping releases of climate-altering greenhouse gases low. Not everyone can afford to live in the city, however. And not everyone wants to. Rings of suburbs have popped up around major cities across the world. Suburbs offer more space, allowing people to build larger homes. Suburbs may offer better schools for a family s kids. But those homes are typically well beyond walking distance from where their owners work, play or learn. So people who live in suburbs often drive long distances. The new findings are an important contribution to climate research, says Matthew Kahn, an environmental economist at the University of California, who was not involved with the study. Kahn would like to see the analysis applied to other parts of the world—Europe, India and China, for instance. That would give scientists a better feel for how culture might mix with location to influence our carbon footprints.

"Footprint" refers to the amount of carbon dioxide released by______.

A．an individual

B．an industry

C．a region

D．a country