1519~1522年麦哲伦船队实现了绕地球一周的航行，证实了地球的形状是（　　）

1949年2月，美国乔治•凯南致国务院电报：“总结起来，我们面对着一个政治力量，它狂热地坚信，它和美国有永久性的妥协办法。如何对付这个力量的问题，无疑是我国外交所要面临的最巨大的任务。……这个问题是我们有能力解决，且不必通过一场全面的军事冲突来解决的。”这个言论体现出来的美国的外交政策是（    ）

根据短文理解，回答问题

    Many students in the USA always take the school bus to school every day. Parents think taking the school bus is very safe. But sometimes it is also dangerous (危险的). Some students die (死亡) because they don't follow the school bus rules. Here are some school bus rules:

◆Arrive at the bus station early.

◆Wait for the bus at the bus station.

◆Don't play around (在周围) the bus.

◆Don't run everywhere at the bus station.

◆Don't walk or run in the bus when it is going.

◆Don't try to leave the bus before it stops.

These rules are good for students. Remember them and follow them. So you can keep yourself safe.