信息高速公路是现代化高科技的产物,其所属的行业是 [ ]
A、邮政通信
B、电信通信
C、交通运输
D、商业贸易
答案:B
The wide spread of super hybrid rice around the world ______ the lives of millions and millions of people suffering from hunger to a great level.
A.has improved
B.is improving
C.improved
D.has been improving
两颗人造卫星A、B绕地球作匀速圆周运动,运动的周期之比为TA:TB=1:27,求:①、两卫星轨道半径之比;②、两卫星运动速率之比。