冠牙体预备的一般顺序是（）
A.颊舌-邻-
B.颊舌--邻
C.-颊舌-邻
D.邻-颊舌-
E.邻--颊舌
参考答案：C
Natural monopolies exist because they can produce at lower costs with greater output, which means there are economies of scale. Which of the following industries is typically a natural monopoly（）
A. Technology.
B. Utilities.
C. Oil.
对可确定作用的自然灾害，既有结构宜通过结构（）评定其抗灾害能力。
A.安全性
B.适用性
C.经济性
D.耐久性
