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?