Trees should only be trimmed when there is a good and clear reason for doing so and, fortunately ,the number of such reasons is

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问题    Trees should only be trimmed when there is a good and clear reason for doing so and, fortunately ,the number of such reasons is small. Trimming involves the cutting away of overgrown and unwanted branches, and the inexperienced gardener can be encouraged by the thought that more damage results from doing it unnecessarily than from leaving the tree to grow in its own way.
   First, trimming may be done to make sure that trees have a desired shape or size. Tile object may be to get a tree of the right height. And at the same time to help the growth of small side branches which will thickens its appearance or give it a special shape. Secondly, trimming may be done to make the tree healthier.  You may cut out diseased or dead wood ,or branches that are rubbing against each other and thus causing wounds. The health of a tree may be encouraged by removing branches that are blocking up the center and so preventing the free movement of air.
   One result of trimming is that an open wound is left on the tree and this provides an easy entry for disease, but it’s a wound that will heal. Often there is a race between the healing and the disease as to whether the tree will live or die. So that there is a period when the tree is at risk. It should be the aim of every gardener to reduce the risk of death as far as possible. It is essential to make the area which has been trimmed smooth and clean ,for healing will be slowed down by roughness. You should allow the cut surface to dry for a few hours and then paint it with one of the substances available from the garden shops produced especially for this purpose. Trimming is usually done in winter, for you can see the shape of the tree clearly without interference  front the leaves and it is, too, very unlikely that the cuts you make will bleed. If this does happen, it is ,of course, impossible to paint them properly.
A special substance is painted on the tree to ______.

选项 A、make a wound smooth.
B、prevent a disease from entering a wound.
C、cover a rough surface.
D、help a wound to dry.

答案B

解析 根据题干中定位到第三段倒数第三句话,根据上下文理解句中提到的for this purpose是指前一句话中提到的to reduce the risk of death,据此选[B]阻止病毒从伤口进入。[A]、[C]、 [D]原文未提及。
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