A、Because they were not easily burned and can last long. B、Because they were very cheap materials. C、Because they could be found

Architecture is to building as literature is to the printed word. The best buildings are often so well constructed that they outlast their original use. They then survive not only as beautiful objects, but as documents of the history of cultures. These achievements are never wholly the work of individuals. (29)Architecture is a social art.
    The revival of classical culture brought about an entirely new age, not only in philosophy and literature but in the visual arts as well. In architecture, the principles and styles of ancient Greece and Rome were brought back to life and reinterpreted. They remained dominant until the 20th century.
    Many kinds of stone are used as building materials. (30)Stone and marble were chosen for important monuments because they are not burnable and can be expected to endure. Stone architecture was often blended with stone sculpture. The use of stone has declined, however, because a number of other materials are more adaptable to industrial use.
    (31) The complexity of modern life calls for a variety of buildings. More people live in mass housing and go to work in large office buildings; they spend their income in large shopping centres, send their children to many different kinds of schools, and when they are sick they go to specialized hospitals and clinics. All these different types of buildings accumulated experiences needed by their designers.
    (31) By the middle of the 20th century, modern architecture, which was influenced by new technology and mass production, was dealing with increasingly complex social needs. Important characteristics of modern architectural works are expanses of glass and the use of reinforced concrete.