A:There
B:This
C:That
D:It
The plane for Yunnan will______ in two hours. Let's go to the airport as soon as possible.
①摩擦生热,②热水袋取暖,③钻木取火.
已知质量分布均匀的球壳对其内部物体的引力为零,设想在地球赤道正上方高h处和正下方深为h处各修建一绕地心的环形真空轨道,轨道面与赤道面共面,两物件分布在上述两轨道中做匀速圆周运动,轨道对它们均无作用力,设地球半径为R,则( )