题目内容：

    Tiny microbes (微生物) are at the heart of a new agricultural technique to manage harmful greenhouse gas. Scientists have discovered how microbes can be used to turn carbon dioxide into soil-enriching limestone (石灰石), with the help of a type of tree that grows in tropical areas, such as West Africa.

Researchers have found that when the Iroko tree is grown in dry, acidic soil and treated with a combination of natural fungi (霉菌) and other bacteria, not only does the tree grow well, it also produces the mineral limestone in the soil around its root.

The Iroko tree makes a mineral by combining Ca from the earth with CO2 from the atmosphere. The bacteria then create the conditions under which this mineral turns into limestone. The discovery offers a new way to lock carbon into the soil, keeping it out of the atmosphere. In addition to storing carbon in the trees' leaves and in the form of limestone, the mineral in the soil makes it more suitable for agriculture.

The discovery could lead to reforestation projects in tropical countries, and help reduce carbon dioxide in the atmosphere in the developing world. It has already been used in West Africa and is being tested in Bolivia, Haiti and India.

The findings were made in a three-year project involving researchers from the Universities of Edinburgh, Granada, Lausanne and Delft University of Technology. The project examined several microbiological methods of locking CO2 as limestone, and the Iroko-bacteria way showed best results. Work was funded by the European Commission under the Future&Emerging Technologies(FET)scheme.

Dr Bryne Ngwenya of the University of Edinburgh's School of GeoSciences, who led the research, said："By taking advantage of this natural limestone-producing process, we have a low-tech, safe, readily employed and easily operating way to lock carbon out of the atmosphere, while improving farming conditions in tropical countries."

1.The passage is mainly introducing ______ .

A.some useful natural fungi and bacteria

B.the soil-enriching limestone created by scientists

C.a newly-found tree in West Africa

D.a new way to deal with greenhouse gas

2.Which of the following is True about tiny microbes?

A.Tiny microbes get along well with the Iroko tree in special soil.

B.CO2 can be broken down by natural fungi and bacteria.

C.The more greenhouse gas is, the more active tiny microbes become.

D.Most tiny microbes like living in dry, acidic soil.

3.What does the underlined word "it" in paragraph 3 probably refer to?

A.Carbon dioxide. B.Soil.

C.Carbon. D.Limestone.

4.According to the passage, what can we infer?

A.The action of the tiny microbes can increase the oxygen in the earth.

B.Researchers tend to use natural power to solve their problem.

C.Researchers have done the experiment on trees in Africa for three years.

D.West Africa is one of the most polluted areas all over the world.