听力原文：

　　A beetle invasion in the United States has killed at least twenty million ash trees. The invasion of the emerald ash borer was first discovered near Detroit, Michigan, in 2002. Experts believe the small green insects arrived in the 1990's in shipments of goods from China.

　　The emerald ash borer has destroyed trees in the Midwest and as Far East in the United States as Maryland. [29]The insects have also spread as far north as Ontario, Canada.

　　Ash trees can resist many diseases. But they cannot resist the emerald ash borer. It lays eggs on the bark. The young larva drill into and feed on the inner bark. This harms the ability of the tree to transport water and nutrition.

　　In some places, farmers are using "detection trees". They have an area where bark has been cut away. The area circles the tree. [30]The process weakens the tree. It makes them easier targets for borers, and shows if the insects are nearby.

　　Efforts to stop the spread of the emerald ash borer include cutting down affected trees. There are worries that the ash tree might disappear unless the invasion is controlled. To prepare for such a possibility, a government laboratory is asking people to send in the seeds from ash trees. [31]It examines and x-rays the seeds to make sure there are no living borer fertilized eggs. Only the best seeds are remained and then sent for storage in a seed bank. There, they are dried and frozen. Should the seeds ever be needed, the hope is that scientists might someday develop an ash tree that could resist the little green attackers.

　　Questions 29 to 31 are based on the passage you have just heard.

　　29. How far do the insects spread northward?

　　正确答案：D

　　解析：短文说，钻孔虫向北扩散至Ontario，Canada。故D是正确答案。四个选项是地名，可猜测题目与地点有关。A和C是美国的州名，B和D是国家名，边听边做好笔记。解题的关键在于听清楚问题，就可以对号入座找出正确的答案了。

　　30. Why is part of the bark cut away?

　　正确答案：C

　　解析：短文说，钻孔虫是在树上产卵子，孵出的幼虫钻到内层树皮吸取营养，故把部分树皮削去会使树更容易成为攻击的目标，当钻孔虫来袭时就能有所发觉。故C是正确答案。从选项可以初步猜测题目与树木和昆虫有关。解题时需听清楚问题问的是什么。

　　31. What is the government laboratory trying to do?

　　正确答案：B

　　解析：短文说，实验室对种子进行检查和照x-ray，确保没有钻孔虫的受精卵，只有的才被保存起来。故答案是B。从选项可以初步猜测题目与措施有关。“Only”表明文章要提出特殊的要点了，要警惕。

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