B

    People often think of the North Pole(北极) and the South Pole as similar frozen(冰冻的) wastelands. They are both places with very cold temperatures and few people live there. However, the North and South Poles are not much alike as people may imagine.

    The North Pole has no land, only thick ice. Temperatures seldom go above 32°F, at which water turns into ice. Most of the time, it stays below zero. In winter temperatures are usually as low as -30°F.

    Although conditions were very poor, people tried for many centuries to reach the North Pole. About 100 years ago, two men, Robert Peary and Mathew Henson, were able to get to this environment full of troubles and difficulties. They reached the North Pole on March 8th, 1909. It was really a hard trip for them.

    There are few things more dangerous than crossing the Arctic(北极圈) on foot. People who stayed there may face many problems: very cold temperatures, sudden storms, and even hunger. Most of the area is uninhabited. Few people can live in such a difficult place.

    People might think that with such low temperatures for most of the year, the ice would be thick and hard. However, this is not true in the Arctic. The movement of the ocean water under the ice may often cause many different changes on the surface. Sometimes the ice breaks into two opening lanes of water(水道) called “leads”. Anyone who falls into a lead can be frozen to death in a few minutes.

在△ABC中，AC=8，BC=6，AB=10，则△ABC的外接圆半径长为（   ）

材料一：15世纪早期，海洋上曾有中国人频繁活动的身影。欧洲人直到1500年前后，才冲破茫茫大海的阻隔，把世界连成一个整体，海洋时代就此开启，然而中国，自……死后，中国人的身影就在海洋上消失了。从此，海洋带给中国人的不再是友谊和荣耀，而是噩梦的代名词，骨肉的离散与悲欢。

材料二：日益严重的海防危机，让当时的中国意识到海军建设的重要性，于是近代意义上的中国水师（海军）诞生了，然而，随着北洋水师的全军覆没，中国水师不仅输给了日本，同时再次导致骨肉分离。

材料三：近几年来，南海问题成为世界关注的焦点。2011年以来，美国为实施亚太再平衡战略，与中国周边国家频繁军演，插手中国南海事务，妄图使南海问题复杂化。