两颗人造卫星A、B绕地球做圆周运动，周期之比为T_A:T_B=1:8，则轨道半径之比和运行速度之比分别为：
A．r_A:r_B ="4:1" , V_A:V_B ="1:2           "   B．r_A:r_B ="4:1" , V_A:V_B ="2:1"
C．r_A:r_B ="1:4" , V_A:V_B ="1:2             " D．r_A:r_B ="1:4" , V_A:V_B ="2:1"