If you want to travel to a new place, what will you worry about first? The answer may be how to1the way.

    In big cities many people go from one place to2by bus, taxi or underground. The bus is a popular3of travelling.  If you want to use a bus  you should (应该） know4bus to take. You should5the timetable (时间表) right so that you can take the6bus and get to your place on time. And it's important for you to know7to get off.

    In the countryside (农村), 8can be much simpler (简单). In some places with small roads, people get to school or their offices by9. In my hometown we usually take a boat to school. In other places, students can go to school10foot because they live near the school.

长城总长约为6700010米，用科学记数法表示是 （  ）.(保留两个有效数字)