Since the earliest times, trees have always been very useful and important to man who made use of them in numerous (众多的) ways for his daily life. Even today trees continue to serve man in so many useful ways. Trees provide man with food, fuel for burning and building material in the form of wood. Without trees it would be impossible to build houses, boats and even bridges. Furniture such as tables, chairs and cupboards are also made from wood. In the tropics (热带) where it is very warm throughout the year, trees protect man from the fierce heat of the sun. They are useful in preventing good and fertile top soil from being washed away during heavy rains which are so common in the tropics. If there are no trees or vegetation (植物) of some sort to hold back the soil with their roots, heavy rains would wash away the rich surface soil so essential for agriculture. The result is that the land becomes useless and unproductive. There are many desert areas in the world. A long long time ago these areas must have been very rich, fertile areas, but because of ignorance people in ancient times cut down trees indiscriminately (杂乱地). They never bothered to plant new trees. Strong winds gradually blew away the rich surface soil and eventually the land turned into useless deserts where nothing could grow. According to the passage, ______.
A. early man did not know how to make use of trees B. trees are not as useful now as they were in the past C. trees were more useful in the past than they are now D. men have always found trees useful