主要用于碳素工具钢、合金工具钢、高速钢和铸铁工件研唐的磨料是（）磨料。
A.碳化物
B.氧化物
C.金刚石
D.石英
参考答案：B
Which of the following functions would the nurse expect to be unrelated to the placenta（）
A. Production of estrogen and progesterone.
B. Detoxification of some drugs and chemicals.
C. Exchange site for food, gases, and waste.
D. Production of maternal antibodies.
下面属于等级扩散的例子是( )。
A．移民过程B．城市对周围农村的影响C．城市化的近域推进D．某些新思想、新技术逐级向下扩散
