阅读理解。
A prairie is a type of grassland. Prairies are made up of a complete interdependent ecosystem. This means
that all kinds of plants and animals in a prairie region depend on one another for survival (生存). The members
of a prairie ecosystem include grasses, flowering plants, animals, soil (土壤), sand, and fire.
You may look at a prairie and think there's nothing there but grass, but that' s not true. Most of the prairie's
living things are below the ground. Underneath the prairie' s surface are roots and bulbs (根茎). The roots of
the prairie flowers and grasses run deep. Some of them are as deep as 3.5 metres below the ground.
It is impmtant for the roots to run deep because fires are a part of the prairie ecosystem. When a fire bums
through the prairie, plants with shallow (not deep) roots can not survive. Plants that have deep root systems
are able to survive the fires. When fire burns across a prairre, it bums dead plants from the top and returns
those nutrients (养料) to the soil. This adds to the richness of the prairie soil. Prairies have some of the
richest soil in the United States. This cycle of growth and buming has been going on for thousands of years.
Big bluestem is the plant that gives parts of the prairie the name "tallgrass prairie".
Big bluestem has very deep roots, and its stems (茎) are often 2 metres tall.
The American bison (野牛) or buffalo, is the country' s largest land animal. Its home is on the prairies. In
1830, there were an estimated 30 million to 60 million bison in the United States. By 1889, Americans had
made the bison population to fewer than l,000 animals. Through great efforts, bison are being reintroduced to
the prairies. They are also kept in zoos. The bison population has grown to several hundred thousand today.
Humans have put the prairie ecosystem in danger. Through constant farming, building, fire prevention,
and ignorance( not knowing) of how the prairie works, the United States has lost much of its natural prairie.
Through education and other work , we can hope to save some of it.
1. A prairie is a type of  ________.
A. plants
B. animals
C. grassland
D. soil
2. What is the meaning of "interdependent ecosystem" in the first line?
A. All things in a prairie region depend on one another.
B. Animals live on plants in a prairie region.
C. Fire plays an important role in a prairie region.
D. Soil has a close relationship with a prairie region.
3. By 1889, the bison population was ________.
A. about 30 million
B. fewer than a thousand
C. about 60 million
D. several hundred thousand
4. We can leam from the article that ________.
A. fires may destroy a prairie and therefore must be prevented
B. the interdependent ecosystem is both below and above ground
C. the prairie ecosystem is in danger because of farming and building
D. people haven't realized that they are losing their natural prairie

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