TOEFL Listening: TPO-TOEFL阅读TPO - 4N4X34410EUN0BTJN$

According to paragraph 4, which of the following is true about materials used in the construction of buildings? A. Because new building materials are hard to find, construction techniques have changed very little from past generations. B. The availability of suitable building materials no longer limits the types of structures that may be built. C. The primary building materials that are available today are wood, stone, and brick. D. Architects in earlier times did not have enough building materials to enclose large spaces.