质量为m的飞机以速率v在水平面上做半径为R的匀速圆周运动，则空气对飞机的作用力为[ ]
A．mv2/R
B．
C．
D．mg
答案：B
It is reported that a man more than 70 hours without water in the ruins after the Qinghai earthquake.
A．had survived
B．survives
C．survived
D．is surviving
下列选项中，属于机械式结构特点的是（）。
A.授权化
B.集权化
C.临时化
D.机械化
