题目

 -_____ of the water _____ polluted. We must try to protect our environment. -I think so.    A. Two third; are   B. Two third; is     C. Two thirds; are    D. Two thirds; is 答案：D已知第一宇宙速度是7.9km/s，如果一颗人造卫星的轨道高度，h=3R地，它的环绕速度应为 [     ]A．7.9 km/s    B．3.95 km/s C．1.98 km/s    D．不能确定