Forests have always been useful and important to man who make use of them in many ways. Every day trees are serving man everywhere. Trees supply man with fruits and building materials in the form. of wood. Without trees it would be impossible to build houses, boats, bridges and so on. Furniture such as desks, chairs and beds is made of wood. Trees can stop man from terrible heat. They're also useful in preventing good and rich top soil from being washed away during heavy rains.
If there were no trees, heavy rains would wash away the rich top soil that is so important to plants. The result is that the land will become a desert. There are plenty of desert areas in the world. A long time ago these desert areas used to be very rich areas, but man in the past had not enough knowledge about science of nature, they cut down too many trees in the area where they lived and never planted new ones. By and by the rich top soil was blown and washed away by strong winds and heavy rains. In the end the rich land changed into useless deserts where nothing could grow.
According to the passage, ______.

A. a long time ago, man did not know how to make use of wood
B. trees are not as useful as they were in the past
C. trees were more found in the past than they are today
D. people have 'always found trees useful