Text 2 for questions number 67-70.

    The tropical rainforest is a forest of tall trees in a region of year-round warmth. An average of 50 to 260 inches (125 to 660 em) of rain falls yearly.

    Rainforests now cover less than 6% of Earth's land surface. Scientists estimate that more than half of all the world's plant and animal species live in rainforests. Tropical rainforests produce 40% of Earth's oxygen.

    A tropical rainforest has more kinds of trees than any other area in the world. Scientists have counted about 100 to 300 species in one 2 1/2 acre (1-hectare) area in South America. Seventy percent of the plants in the rainforest are trees. 

    About 1/4 of all the medicines we use come from rainforest plants. Curare comes from a tropical vine and is used as anesthetic and to relax muscles during surgery. Quinine, from the cinchona tree, is used to treat malaria. A person with lymphocytic leukemia has a 99% chance that the disease will go into decrease because of the rosy periwinkle. More than 1,400 varieties of tropical plants are thought to be potential cures for cancer.

    Each of the three largest rai nforests - the American, the African, and the Asian - has a different group of animal and plant species of the other two rainforests. In addition, different areas of the same rainforest may have different species. Many kinds of trees that grow in the mountains of the Amazon rainforest do not grow in the lowlands of that same forest.

What is the topic of the passage?