什么是流量标定中的称量法？
参考答案：
在测量时间内测出流入标准容器内的流体质量，以求得流量的方法。
At the beginning of class, the raise of desks could be heard outside the classroom. [ ]
A. opened and closed
B. to be opened and closed
C. being opened and closed
D. to open and close
社会结构的核心是社会阶层结构，其标志性指标是（）的比重。
A.工薪阶层
B.中产阶层
C.高收入阶层
D.低收入阶层
