1.等他到机场时，飞机已经起飞了。
2.在校园里，车速被限制每小时30英里。
3.这对双胞胎姐妹之间的不同之处在于，一个依赖父母，另一个却很自立。
4.一方面汽车有用，可另一方面它们也造成污染。
參考答案:1. By the time he got to the airport, the plane has already taken off.
2.In school, driving speed is limited to 30 miles per hour.
3.The difference between the twin sisters is that one is depending on her parents, and the other is on her own.
4.On one hand, cars are useful, on the other hand, they produce pollutions.
