单选题
Disposing of waste has been a problem since human started producing it. As more and more people 【C21】______to live close together in cities, the waste-disposal problem becomes increasingly difficult. During the eighteenth century, it was【C22】______for several neighboring towns to get together to select faraway spot as a dumpsite. Residents or trash haulers would transport household rubbish, rotted wood, and old possessions to the site. Periodically some of the trash was burned and the rest was buried. The unpleasant sights and smells caused no problem【C23】______nobody lived close by. Factories, mills, and other industrial sites also had waste to be disposed of. Those【C24】______ on rivers often just dumped the unwanted remains into the water. Others built huge burners with chimneys to deal with the problem. Several facts make these choices unacceptable to modern society. The first problem is space. Dumps, which are now called landfills, are most needed in heavily【C25】______areas. Such areas rarely have empty land【C26】______for this purpose. Property is either too expensive or too close to residential neighborhoods. Long-distance trash hauling has been a common practice, but once farm areas are 【C27】______to accept rubbish from elsewhere, cheap land within trucking distance of major city areas is almost nonexistent. Awareness of pollution dangers has 【C28】______in more strict rules of waste disposal. Pollution of rivers, ground water, land and air is a【C29】______people can no longer pay to get rid of waste. The amount of waste, however, continues to grow. Recycling efforts have become commonplace, and many town require their people to take part. Even the most efficient recycling programs, 【C30】______, can hope to deal with only about 50 percent of a city's reusable waste.