听力原文: Trees should only be pruned when there is a good and clear reason for doing so and the number of such reasons is small. Pruning 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, pruning may be done to make sure that trees have a desired shape or size. The 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 thicken its appearance or give it a special shape. Secondly, pruning may be done to make the tree heavier. You may cut out diseased or dead wood, or branches that are rubbing against each other and thus cause wounds. Removing branches that are blocking up the center and so preventing the free movement of air may encourage the health of a tree.
One result of pruning is that an open wound is left on the tree and this provides an easy entry for disease, but it is 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 that risk of death as far as possible. It is essential to make the area, which has been pruned 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 garden shops produced especially for this purpose. Pruning is usually done in winter, for then you can see the shape of the tree clearly without interference from the leaves and also it is very unlikely that the cuts you make will bleed. If this does happen, it is, of course, impossible to paint them properly.
Why should pruning be done?
A. To make the tree grow taller.
B. To improve the shape of the tree.
C. To get rid of the small branches.
D. To make the small branches thicker.
正常人的脉搏平均为72次/分,现测得10例铅中毒患者的脉搏(次/分)如下54,67,68,78,70,66,67,70,65,69欲比较正常人与铅中毒患者的脉搏有无区别,应采用
A. 两样本率比较的t检验
B. 两样本均数比较的t检验
C. 配对设计的t检验
D. 四格表表检验
E. 样本均数与总体均数比较的t检验
对一般企业而言,下列项目可视为现金及现金等价物的是()。
A. 委托贷款
B. 可提前支取的定期银行存款
C. 银行承兑汇票
D. 购入准备持有1个月即出售的股票