一朵桃花的结构中与果实和种子形成有直接关系得结构是[ ]
A.花萼
B.花瓣
C.雌蕊和雄蕊
D.花托
C
About 170 people _____ and dozens more _____ after a flood hit an Indonesian town last week.
A.were killed; are still missing
B.have been killed; were still missing
C.are killed; are still missing
D.had been killed; were still missing
已知地球半径为R,地球表面重力加速度为g,万有引力常量为G,现有一质量为m的卫星绕地球做匀速圆周运动,其轨道半径为r,求:
(1)地球的质量M
(2)卫星在轨道上做匀速圆周运动的周期