A:①②③
B:①②④
C:①③④
D:②③④
We should never ____ (pollution) water because it is not easy for it to come to our flats.
继“天宫”一号空间站之后,我国又发射“神舟八号”无人飞船,它们的运动轨迹如图所示。假设“天宫”一号绕地球做圆周运动的轨道半径为r,周期为T,万有引力常量为G。则下列说法正确的是( )