青少年的生理压力来源有（）。

Alaska, which was called Russian America before it was sold to the United States of America, joined the union as the forty-ninth state in 1959. Alaska is now the largest of all the 50 states of the United States. It was in 1867 that President A. Johnsons Secretary of State(国务卿), Seward bought Alaska from the Russians at a cost of 7. 2 million. The buying of the huge northern land mass seemed at first something foolishly done. Not only was Alaska difficult to reach, but it was also hard to live in, and it appeared to have no importance in time of war. Besides, there are volcanoes there as Alaska lies on the Pacific ' ring of fire'(火山带). In Alaska large treeless areas are covered with snow all the year. For these reasons the buying of Alaska was called 'Sewards Fooly' at that time. However, in 1896 gold was found in Alaska, and people poured into the land quickly. Since then other important natural resources were discovered, including oil. Soon people changed their thinking about 'Sewards Fooly'. But most people visit Alaska in order to see the endless beauty of nature that the northern land discloses to them. For instance, there are about 11, 000 islands in Alaska. And in a certain area of Alaska the sun does not set for 82 days every year. 点击查看答案

根据思维任务的性质、内容和解决问题的方式，可以把思维分为( )。 点击查看答案