A．硬币
B．盐水
C．橡胶棒
D．人体
答案：C
两颗人造地球卫星A、B的质量之比mA：mB=1：2，轨道半径之比rA：rB=1：3，某一时刻它们的连线通过地心，则此时它们的线速度之比vA：vB=______，向心加速度之比aA：aB=______，向心力之比FA：FB=______．
What do we know about Susan
A．She learned Spanish in America.
B．She doesn’t know Spanish.
C．She improved her Spanish in Mexico.
D．She knew Spanish before going to Mexico.
