Read the passage:

Major cities all over the world are changing color. The tall buildings that make up the landscape of these urban environments are becoming greener. People are trying to help the environment and have found that green roofs are one way to do so. By planting grass and small shrubbery on the tops of buildings, the environment is getting a helping hand.

It is more expensive to build a green roof than it is to lay asphalt or shingles on a roof, but the trend is growing. The basic green roof requires a frame, waterproof membrane, gravel for drainage, fibrous material to retain moisture, soil for planting, and a barrier that prevents roots from penetrating the building.

Green roofs started in Europe and are now gaining a strong foothold in the United States. The largest green roof in New York City is built on the roof of a processing and distribution facility. It is actually the biggest green roof in the country. Being in New York, it has faced snow, rain, freezing, and thawing. It has survived it all.

The city of Chicago, Illinois, 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, not only to the environment, but also to 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.

Select the reason the author included the point that it is expensive to build a green roof.

a
It contrasts the fact that gravel is needed.

b
It contrasts the fact that the trend is growing.

c
It is a comparison between price and shape.

d
It is a comparison between price and size.