Trees should only be pruned when there is

题目：

Trees should only be pruned when there is a good and clear reason for doing so. And fortunately, the number of such reasons is small. Pruning involves the cutting away of over grown 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 it sown way.

Firstly, pruning may be done to make sure that trees have 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 healthier. You may cut out diseased or dead wood, or branches that are rubbing against each other and thus cause 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 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 important 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 materials 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.

Trees become unhealthy if the gardener （）.

A．to helps a wound to dry

B．to covers a rough surface

C．damages some of the small side branches

D．allows too many branches to grow in the middle