A project likely to evolve in the near or intermediate future is space tourism. Today space tourism has become a pure commercial

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问题     A project likely to evolve in the near or intermediate future is space
tourism. Today space tourism has become a pure commercial activity【S1】________
that would not have to rely unpredictable government decisions. It would【S2】________
therefore put space operations on a much more solid, dependable footing
and would enormously expand man’s space activity. If some people see【S3】________
space tourism as a potential multibillion—dollar business, difficult
technical problems remain to be solved.
    First, in order to fulfill the dream of space tourism, a manned
reusable rocket like the Venture Star must be built. Then, the second【S4】________
serious problem must be addressed:Even today’s most—advanced space
rockets are able to lift only about 2. 5%of their launch weight into
0rbit—a ratio that has hardly changed in the last 40 years. But it is【S5】________
simply impossible to find other alternations other than a rocket. This【S6】________
unsatisfying fact is a result of the limits of material characteristics and
chemical power sources, and there is no indication which any【S7】________
fundamental breakthrough call be expected in these areas in the
foreseeable future. Then these who have long-cherished dreams of space【S8】________
tourism have to Wait forever.
    Operational costs may be somewhat reduced by the use of air-
breathing engines to help a reusable rocket penetrate into the earth’s
atmosphere. Today the highest cost attainable to lift a payload into orbit【S9】________
is more than$4, 000 per pound;the price of a ticket to space would
have to be at least$50. 000. It is questionable whether a sufficient
number of customers could be tempted to pay SO a high price for a short【S10】________
trip around the world.  
【S4】

选项

答案第二个the—a

解析 冠词误用。the second表特指,意为排序中的“第二个”。a second表泛指,意为“另一个,又一个”。此处句意为“另一个严重的问题也应得到解决”,故a second符合句意。
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