垄断在经济生活中占统治地位,是进入()最重要的经济特征。
参考答案:帝国主义
They are very familiar with this kind ofbusiness, so there's no need to be______about the outcome. [ ]
A. enthusiastic
B. optimistic
C. concerned
D. controversial
人造地球卫星在距地球表面高度等于地球半径3倍处做匀速圆周运动.已知地球半径为R,地球表面的重力加速度为g,求:
(I)卫星的线速度;
(2)卫星围绕地球做匀速圆周运动的周期.