第13题: [单项选择]Questions 19 to 21 are based on the passage you have just heard.
A. It is an important measure to retain genes. B. It is the only way to maintain biodiversity. C. Good harvests can be ensured. D. Gardeners can be protected from disasters. 参考答案:A 答案解析:[听力原文]
Why are the gardeners encouraged to grow traditional plants
第21题: [单项选择]调和模型石膏时,调拌速度过快,将导致 A. 强度增加,膨胀减少 B. 强度降低,膨胀增加 C. 强度增加,膨胀增加 D. 强度降低,膨胀减少 E. 硬度增大,强度增加 参考答案:B 答案解析:[解析] 模型石膏的固化是半水石膏向二水石膏晶体转变的过程。调和速度过快,则会产生过多的结晶中心,使晶体的生长相互挤压,产生膨胀;同时由于晶核数多,晶体细小,材料的强度也相应降低;另外过快的调拌还会带