在确定材料消耗量时，通常采用的方法包含( )。
A．测算法
B．观察法
C．实验室试验法
D．比较法
参考答案：D
The little girl ,but her family was too poor at that time.
A．could go to school
B．could have gone to school
C．must have gone to school
D．can have gone to school
叙述离心机开、停车操作原则，为什么？
