There is no doubt about it: the Earth really is growing warmer. If you live in a city, tha

t is, urban【C1】______ have known for years that swaths of asphalt and concrete tend to【C2】______ the summer heat. As cities have grown,【C3】______ has the temperature problem. This summer, NASA and the Environmental Protection Agency teamed up with several U.S. cities to study urban "heat islands" and【C4】______ ways to bring some【C5】______ . The pilot program【C6】______ satellite, airplane and ground-based date to produce detailed maps of the hot and (comparatively) cool zones in the【C7】______ areas.

Images of Salt Lake City,【C8】______ in late July, document the【C9】______ of the urban heat problem. Some dark rooftops reach a temperature of 160 degrees F.(71 degrees C.). Jeff Luvall,【C10】______ heads the heat island project, notes that use of【C11】______ materials for roofs and parking lots would help【C12】______ solar heat upward. Urban forests also help reduce temperatures,【C13】 ______ the extent of the improvement depends on both the extent and arrangement of the forests. Luvall reports that【C14】______ along streams brings cool air into Salt Lake City; developers who turn these streams into concrete ditches throw away a【C15】______ natural air conditioning system.【C16】______ , scientists continue to fret about the broader, potentially【C17】______ more troublesome problem of global warming. Vice President Al Gore recently cited government data showing that the first half of 1998 was the warmest year【C18】______ record and called on Congress to adopt【C19】______ that would【C20】______ emissions of greenhouse gases.

【C1】

A．population

B．citizens

C．dwellers

D．people