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．