从层流转变为紊流时的雷诺数称为（）。
A.上临界雷诺数
B.下临界雷诺数
C.临界雷诺数
D.动力粘度
参考答案：A
Some parents are just too protective. They want to （）their kids from every kind of danger, real or imagined.
A. spot
B. dismiss
C. sheher
D. distinguish
简述水泵结合器的作用？
