Solar energy is not widely used ___________ it is friendly to the environment.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

      The green building movement, started in the 1970s as a way to reduce environmental destruction, is changing the way buildings are constructed. In the early years, green builders were a small minority, and their goals of reducing the environmental impact of buildings were considered unrealistic. Now, however, the movement is growing, as builders have been able to take advantage of new technology.

      Green builders try to make use of recycled materials, which means less waste in dumps. Also, they reduce environmental impact by reducing the energy requirements of a building. One way is to provide an alternative, non-polluting source of energy. First, with solar panels, it is possible to produce electricity from the rays of the sun. Once installed, they provide energy at no they cost and with no pollution. 

      Another solution is to reduce the amount of energy required in a building. It is possible to cut electricity use noticeably by improving natural lighting and installing low-energy light bulbs. To reduce the amount of fuel needed for heating or cooling, builders also add insulation to the walls so that the building stays warmer in winter and cooler in summer. 

      One example of this advanced design is the Genzyme Center of Cambridge, the most environmentally responsible office building in America. Every aspect of the design and building had to consider two things: the need for a safe and pleasant workplace for employees and the need to lessen the negative environmental impact. 75 percent of the building materials were recycled materials, and the energy use has been reduced by 43 percent and water use by 32 percent, compared with other buildings of the same size. 

      In other parts of the world, several large-scale projects have recently been developed according to green building principles. One of these is in Vauban, Germany, in an area that was once the site of army housing. The site has been completely rebuilt with houses requiring 30 percent less energy than conventional ones. These houses, heated by special non-polluting systems, are also equipped with solar panels. 

      A larger project is under way in China. The first phase of this project will include houses for 400 families built with solar power, non-polluting bricks, and recycled wall insulation. In a second phase, entire neighborhoods in six cities will be built. If all goes well, the Chinese government plans to copy these ideas in new neighborhoods across China. 

      Green building ideas, on a small or large scale, are spreading. Individuals, companies, and governments are beginning to see their benefits. Not only are they environmentally friendly, green buildings improve living and working conditions and also save money in the long run. 

(Adapted from "Advanced Reading Power" by Beatrice S. Mikulecky and Linda Jeffries)

Which of the following does the passage mainly discuss?