2. 外语
  3. In 1961, NASA has encouraged studies of space-planes that could be built using existing technology. The main difficulties of sp

In 1961, NASA has encouraged studies of space-planes that could be built using existing technology. The main difficulties of sp

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问题 In 1961, NASA has encouraged studies of space-planes that could be built using existing technology.
The main difficulties of space flight are neither______ nor______.
选项
答案technical，commercial
解析 第2段的第l句可以找到答案，要实现这一梦想并不是技术和商业上的问题。
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大学英语六级

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