A、Establishing energy stations in space. B、Finding a star similar to the earth. C、Developing a kind of manned spacecraft. D、Find

The first flight of the Space Shuttle Columbia in the spring of 1981 was a revolutionary development in space exploration. Unlike the rockets of the previous twenty years, Columbia has the enormous advantage of being specially designed to return to earth and make further flights. Among the large number of projects, scientists are investigating its future use. The most exciting, perhaps, is that the first steps can now be taken towards establishing human colonies in space. The present Space Shuttle can only stay in space for about a week, but it could take people out to build "islands" where they stay for longer periods. By the year 2050 scientists estimate that man will be able to construct permanent settlements.
    The first "islands" would be energy stations. There would be no hope of establishing colonies in space unless people could obtain energy there, instead of getting it from the earth. However, if the original could convert solar energy into electricity, it would be possible to provide the equivalent of a filling station for cars on earth, allowing the Space Shuttle to refuel and stay in orbit longer periods of time.
    If this initial project was successful, much larger solar energy stations would be constructed later with materials brought out by the Shuttle.
    Eventually, man would be able to construct an "inhabitable planet" in space.
Questions 19 to 21 are based on the passage you have just heard.
19. What can we learn from the passage about the Space Shuttle Columbia?
20. What is the big advantage of the Space Shuttle Columbia?
21. What is the basic requirement for establishing colonies in space?