A、Acquire information from satellites more efficiently. B、Realize full coverage of area the satellite passes over. C、Build more

American scientists are developing maps that can identify areas of future flooding. The maps also may help scientists estimate the possibility of extremely dry weather and severe storms.
    Tom Jackson of the United States Department of Agriculture prepared the map after the April flood. He used information provided by satellites in Earth orbit. The maps are produced from dish receivers on the satellites. Each dish receiver measures the wetness of the soil by capturing the natural release of" microwave radiation from the soil. The receivers turn rapidly. This provides complete coverage of each area the satellite passes over.
    Mr. Jackson and other scientists have tested Aqua’s equipment in airplanes and satellites flying over several states. The scientists used the tests to develop ways to change the information gathered into maps.
    Mr. Jackson now plans to compare the microwave dish receiver findings with information from tests on Earth’s soil. He will use instruments at Agricultural Research Service centers in four states. He also will study information provided by forty ground observation centers across the country.
Questions 20 to 22 are based on the passage you have just heard.
20. Why American scientists are developing these maps?
21. How can the satellites catch the information about the soil?
22. What is the future work planned to do?