Are we at the beginning of another Age of Exploration? Perhaps even more important are we

Are we at the beginning of another Age of Exploration？ Perhaps even more important, are we at the beginning of【C1】______ Age of Colonization？ As the population of the world increases towards the point【C2】______ the earth can no longer support all the people【C3】______ on it, the second question becomes urgent. Will we discover a new world,【C4】______ Columbus did, on which human life will be possible？ At this point in the space age, no one can really answer these questions. We can say,【C5】______ , that we will not see tomorrow the kind of space travel that【C6】______ fiction and the movies have shown. It will be a long time before we have flight that run【C7】______ to human colonies on the moon or on one of the planets. We are not even going to be able to【C8】______ immediate advantages of the minerals that we may find on the planets【C9】______ our own solar system.

Great problems must be solved【C10】______ we could send colonies out into place. The distances that must be【C11】______ and the length of time it takes to do that can hardly be【C12】______ . There arc also dangers that we still do not really understand--from radiation, for example, or from pieces of matter 【C13】______ in space, or from contamination from forms of life that might【C14】______ there. There is also need for humans to take their own environment into space【C15】______ them. So far no "island" has been discovered in space on which people can exist without systems that【C16】______ life, and these systems must accompany any future space【C17】______ . Finally, on the most practical【C18】______ , there has to be enormous expense【C19】______ in space exploration. The U.S. and Russian governments have already spent billions of dollars for projects【C20】______ which they can receive a return only in knowledge and not in money.

【C1】

A．another

B．an

C．the

D．one