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阅读下列材料,回答问题。

分析图表,提取两项影响中国古代都城选址的因素,并结合所学知识予以说明。

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信息一:中国古代都城选址受到经济中心影响。

说明:唐代以前,全国的经济中心在关中地区与山东(崤山、函谷关以东)地区,因此,这一时期都城主要集中在西部地区。宋代以后,随着经济重心南移,政治中心也逐渐南移、东移,因此,这一时期都城主要集中在东部地区。

信息二:中国古代都城选址受到外部威胁影响。

说明:唐朝以前中原王朝的外部威胁主要来自西北,因此,都城偏于西北,有利于抗敌自保,周秦汉唐都是如此。元代以后,北方少数民族政权的崛起与巩固北方边境的需要,影响到都城位置选择,因

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    The insects are important to farmers. When a honeybee lands on a flower in plant, pollen (花粉) sticks to its legs. When the bee lands on another flower, some of the pollen falls off and fertilizes the second plant.

    The act of spreading pollen is responsible for many fruits, vegetables, nuts and other crops. Yet about 30 percent of honeybees in the United States and other areas have died in recent years.

    Mike Leggett is a researcher at the University of Maryland. He wants to learn why so many bees are dying. “The number of colonies (蜂群) that die every winter has been one in three. So on average 30 percent of the colonies have died every winter over the last six winters. And that's a huge number.” His research team examined the pollen that honeybees carried to their homes. They found that the pollen contained high levels of 35 different pesticides (杀虫剂), which are chemicals used to protect plants. “Pesticides are used, and have been used, pretty widely, for centuries, to protect plants from diseases,” says Mr Leggett.

    Keith Oh linger, a farmer and beekeeper in Maryland, has watched many of his bees die every winter. Mr Ohlinger thinks widespread bee death is caused by several things happening at once. But he does not feel sure that pesticides are a part of the problem, “I felt it was the result of a lot of little things. I didn't feel that there was probably one smoking gun. But there's a division (分歧) there, some people feel that it is just one thing.”

    Honeybees are important to agriculture. This makes the search for an answer to their death very important for Mr Leggett's team. As he knows, one in every three bites of food we eat is somehow pollinated by honeybees.