统计在研究社会经济现象的数量表现时，常应用到许多概念，如总体和总体单位、标志和指标、变异和变量、指标体系等。
参考答案：对
As a ______, he tries to give the firsthand information of the missing plane to the public as soon as possible.
A．secretary
B．passenger
C．professor
D．reporter
简述旋涡泵的特点？
