2. 外语
  3. In 1961, NASA has encouraged studies of space-planes that could be built using existing technology. The X-34 has a reusable low

In 1961, NASA has encouraged studies of space-planes that could be built using existing technology. The X-34 has a reusable low

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问题 In 1961, NASA has encouraged studies of space-planes that could be built using existing technology.
The X-34 has a reusable lower stage and an expendable upper, the goal of designs is______.
选项
答案to reduce the cost
解析 文章第6段第1句，X-34运载火箭有上下两级火箭组成，这样设计的目的是减少发射小型卫星的成本。
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大学英语六级

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