[单选题]电气化铁路电力机车受电弓与接触点的移动距离D为线路外轨起道高度h的（ ）倍。
A.1～2
B.2～3
C.3～4
D.4～5

Why do people in all countries ride buses to school, to work, and to far places Buses can go where trains and airplanes cannot. They do not cost as much to travel on. Even though they may not move as fast as airplanes and trains, they sometimes take people between two places faster. This is be- cause they may travel a more direct way, or may leave more often, or at better times.
Buses come in different sizes. A small bus can carry only eight or ten people. A large one might have seats for fifty to seventy people and have standing room for more.
Thousands of yellow school buses carry millions of children to school every year. Most of these buses have seats placed quite close together to fit in as many children as possible. Some schools buy their own buses. Others pay to use buses that belong to bus companies.
City and town buses carry people for short distances. The seats of these are close together, and there is standing room as well. Most city bus
A. short-distance buses
B. direct buses
C. school buses
D. long-distance buses