5.
In the hot areas,trees protect man from the terrible heat.They are also useful in keeping rich topsoil(86) ________ being washed away during heavy rains.If there were no trees to hold back the soil with their roots,heavy rain would wash away the rich surface soil,(87)________ will result in the land becoming a desert.A long time ago these desert areas must have been very rich fertile areas,(88) ________ since our ancestors did not have enough knowledge about science of nature,they cut down all the trees in the areas where they lived and never planted new(89)________.By and by,the rich surface soil was blown away and washed away by strong
winds and heavy rains.In the end the land was turned(90)________useless deserts where nothing could grow.