Questions 57 to 61 are based on the following passage.

　　Here amid the steel and concrete canyons, green grass grows. A hawthorn tree(山楂树) stands in new soil, and freshly dug plants bend in the wind.

　　But Chicago City Hall here seems an unlikely spot for a garden of any variety—especially 20,000 square feet of gardens—on its roof.

　　As one of a handful of similar projects around the country, the garden is part of a $1.5 million demonstration projected by the city to reduce its "urban heat islands", said William Abolt, the commissioner of the Department of Environment.

　　Heat islands-dark surfaces in the city, like rooftops-soak up heat. The retention can bake a building, making it stubborn to cooling.

　　The roof of City Hall, a 90-year-old gray stone landmark on LaSalle Street in the heart of downtown, has been known to reach temperature substantially hotter than the actual temperature on the street below.

　　The garden will provide greenery and shade. "And that," said the city officials, "will save the city dollars on those blistering summer days." The project savings from cooling is about $4,000 a year on a new roof whose life span is about 50 percent longer than that of a traditional roof.

　　The sprawling open-air rooftop garden is being carefully built on a multi-tiered bed of special soil, polystyrene, egg-carton-shaped cones and "waterproof membrane" mall to keep the roof from leaking, or caving under the normal combined weight of soil, rain and plant life.

　　The design calls for soil depths of 4 inches to 18 inches. When the last plants and seedlings are buried and the last bit of compost is laid, the garden will have circular brick stepping-stones winding up to hills.

　　"The primary focus of what we want to do was to establish this laboratory on the top of City Hall to get people involved and understanding their impact on the environment and how the little things can make an impact on the quality of life", Mr. Abolt said, adding that the plants also help to clear the air.

　　Rooftop gardens, in places where concrete jungles have erased plants and trees, are not new, not even in Chicago. Arms of greenery dangling over terraces or sprouting from rooftops, common in Europe, are becoming more so in the United States as people become increasingly conscious about the environment.

　　Richard M. Daley, who urged the environmental department to look into the project after noticing rooftop gardens in Hamburg, Germany a few years ago, has praised the garden as the first of its kind on a public building in the country.

　　It will hold thousands of plants in more than 150 species-wild onion and butterfly weed, sky-blue aster and buffalo grass-to provide data on what species adapt best. Small plants requiring shallow soil depths were chiefly selected.

　　57. The rooftop garden project ________.

　　[A] is common and popular in the country

　　[B] is a demonstration project and costs the city government 1.5 million dollars