【单选题】

Trees should only be pruned (修剪) when there is a good reason for doing so. Many gardeners believe 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. The object may be to get a tree of the right height, and 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 healthier. You may cut out diseased or dead wood, or branches that are rubbing against each other and thus causing wounds.A、tree may grow healthier by removing the branches that are locking up the centre and so preventing the free movement of air.
One result of pruning is that an open wound is left on the tree and this provides an easy entry (进入) for diseases, 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. Pruning is usually clone in winter, for then you can see the shape of the tree clearly without the interference from the leaves.
The writer’s purpose when writing this passage is______.A．to describe how pruning a tree is done
B．to discuss different ways of pruning trees
C．to give practical instructions for pruning a tree
D．to explain how a tree wound gets diseased and heals