题目

What’s your favorite transportation? Last week, we did a survey about   16   kinds of transportation. We asked students about their favorite kinds of trasportation. Here   17   some of the results. Most students come to school by bike   18   they like riding bikes. Of course, bikes are cheaper than cars and buses. Some students like cars because they are  19   than bikes. Cars are 20   faster than bikes and buses.   21   some students,   22   are not popular, because they ofen have   23   passengers. Buses are also dirtier and   24  . However, not man19．同步卫星离地心的距离为r，运行速度为V1，加速度a1，地球赤道上的物体随地球自转的向心加速度a2第一宇宙速度为V2，地球的半径为R，则（　　）A．$\frac{{a{\;}_1}}{a_2}=\frac{r}{R}$B．$\frac{a_1}{a_2}=\frac{R}{r}$C．$\frac{V_1}{V_2}=\sqrt{\frac{R}{r}}$D．$\frac{V_1}{V_2}=\frac{R^2}{r^2}$