A
It's known to all that rainforests are important. But do you know why?
Tropical rainforests took between 60 and 100 million years to evolve (进化) and are believed to be the oldest and most complex landbased ecosystem on earth, containing over 30 million species of plants and animals. That's half of the Earth's wild animal species and at least twothirds of its plant species!
Rainforests store water. In fact, it is believed that the Amazonian forests alone store over half of the Earth's rainwater! Rainforest trees draw water from the forest floor and release it back into the atmosphere. Without rainforests continually recycling huge quantities of water, feeding the rivers, lakes and irrigation systems, droughts would become more common.
Do you know that we also depend on trees to cleanse (净化) our atmosphere? They absorb the carbon dioxide that we breathe out, and provide the oxygen we need. However, when rainforest trees are burnt, they release carbon dioxide, which pollutes the atmosphere and contributes to global warming.
Surprisingly, soil in the rainforest is very poor in nutrients. This is because the nutrients are stored in the huge numbers of trees and plants rather than in the soil. Tree roots bind the soil together, while the canopy (树荫) protects the soil from heavy rain. When a tree dies and its trunk falls to the forest floor, it breaks down and the nutrients it contains are recycled. However, if trees are removed from the forest, the nutrients are removed with it. What's worse, the unprotected soil is then simply washed away by heavy rain, causing floods in lowland rivers while leaving upland rivers dry.
Many local people have been living in harmony with the rainforest for thousands of years, depending on it for their food, shelter and medicines. When oil and logging companies come to remove vast areas of forest, they bring diseases which people there have no resistance to and threaten their survival. Often they are also forced to move away from their homes to unfamiliar places, sometimes even being killed in the process.
21.What can we learn about rainforests from Paragraph 2?
A.They are impossible to recover once damaged.
B.They are too old and complex to evolve themselves.
C.They have fewer wild animal species than plant species.
D.They're important homes to wild animal and plant species.
22.Why does the author mention the poor soil in the rainforest?
A.To stress the importance of trees to soil.
B.To encourage improvement of soil there.
C.To explain a very interesting phenomenon.
D.To raise people's awareness of protecting soil.
23.What can we learn from the passage?
A.We can burn rainforests to make the land richer.
B.We should do our best to prevent global warming.
C.We should protect rainforests instead of destroying them.
D.We can depend on many things to cleanse our atmosphere.
