Major cities all over the world are changing color. The tall gray buildings that make up the landscape of urban environments are becoming greener. They aren't getting a fresh coat of paint. Instead, their roofs are being covered in grass and small shrubbery. Many rooftops are beginning to look like nature parks. This change isn't just for fun. It is an attempt to add plants that can clean city air, absorb waste water, and generally add beauty to the city environment.

Green roofs started in Europe and are now gaining a strong foothold in the United States. They are a way to add a little nature to the city environment. People plant vegetable gardens, flowers, and decorative grasses on their rooftops. One green roof in New York City is built on top of a processing and distribution facility. It is the largest green roof in the country. It has survived snow and rain, given employees a beautiful place for a lunch break, and helped clean the city air.

It is more expensive to build a green roof than it is to lay asphalt or shingles. The basic green roof requires a frame and a waterproof membrane. It also requires gravel for drainage. Special material needs to be laid to retain moisture. Of course, soil for planting and a barrier that prevents roots from penetrating the building are also necessary materials for a green roof. Despite the extra cost, the trend is growing.

The city of Chicago now has more green roofs than any other U.S. city. There are 7 million square feet of green roofs planted across the city. Toronto, Canada, is the first city in the western hemisphere to mandate green roofs on all new tall buildings. Green roofs are beneficial to the environment and the cities that build them. A recent study in Toronto suggested that if 75 percent of the flat roofs were greened, the city could save $37 million a year on storm water management and energy bills. If this is true, we may begin to see more and more green added to cities around the world.

What evidence does the author use to support the idea that green roofs are beneficial despite their cost?

A. It is more expensive to build a green roof than it is to lay asphalt or shingles on a roof.
B. Green roofs started in Europe and are now gaining a strong foothold in the United States.
C. The city could save $37 million a year on storm water management and energy bills.
D. Toronto, Canada, is the first city in the western hemisphere to mandate green roofs.