Have you ever entered a tropical rainforest（热带雨林）?Rainforests make up only a small part of the Earth's surface, about six percent, but there are more kinds of trees than any other area in the whole world.
Tropical rainforests are in Africa, Asia, Central and South America, Australia, and on the Pacific islands. The largest tropical rainforest is the Amazon （亚马逊）Rainforest in South America, which covers 1.2 billion acres, or almost five million square kilometres. The second largest rainforest is in Western Africa.
Rainforests provide us with many things. In fact, the Amazon Rainforest is the "lungs of our planet" because it produces 20% of the world's oxygen（氧气）. One fifth of the world's fresh water is also found in the Amazon Rainforest. Furthermore, one half of the world's species of animals, plants and insects live in the Earth's rainforests. 80% of the food we eat first grew in the rainforest. 25%of the drugs we take when we are sick are made of plants that grow only in rainforests. Some of these drugs are even used to fight and cure cancer.
There are four different levels of trees in a rainforest. The forest floor is the lowest layer full of animal life and receives the least amount of light. The under storey is the dark, cool environment under the leaves but over the ground. The canopy layer is the upper parts of most of the trees. The emergent layer contains a small number of very tall trees.
These different parts of a rainforest exist together to create an ecosystem for many animals, plants and human beings.
B . 
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D . 
