2. 外语
  3. Someday, in【C1】______ the outer planets of our solar system, humankind will want to do【C2】______ send small probes that merely

Someday, in【C1】______ the outer planets of our solar system, humankind will want to do【C2】______ send small probes that merely

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问题      Someday, in【C1】______  the outer planets of our solar system, humankind will want to do【C2】______ send small probes that merely fly rapidly by them. In time, we will want【C3】______  spacecraft that will go【C4】______ orbit around these alien planets, land robots on their moons, and even return rock and soil sample back to Earth.【C5】______ , we will want to send humans to their moons, on at least【C6】______  which liquid water—the fundamental requirementfor life【C7】______  we know it—is believed to be【C8】______ .
     For missions such as these, we will need rockets powered by a nuclear reaction【C9】______ igniting chemicals. Chemical rockets have served us well. But the【C10】______ low amount of energy that they can deliver for a given mass of fuel is a severe【C11】______ when dispatching spacecraft【C12】______ long distances. To reach the outer planets, for example, a chemically-powered space vehicle must【C13】______ fuel by having a very small mass and making【C14】______ use of gravity "assists", in which the craft maneuvers close enough to a planet for the planet’s gravity to propel the craft forward,【C15】______ its speed.
     In technical term, chemical rockets have low【C16】______ velocity growth, which means that their exhaust velocities are not high enough to【C17】______ very high speed to the rocket. Nuclear rockets,【C18】______ their chemical counterparts, could impart a maximum velocity of【C19】______   about 22 kilometers a second. Such a high velocity would make possible a direct path to Saturn (土星), reducing travel time from about seven years to as【C20】______ as three.  
【C7】
选项 A、as
B、that
C、what
D、which
答案A
解析 结构衔接题。本处意思是：正如我们知道的那样，液态水是生命的基本需要。as引导非限定性定语从句，意思是“正如，像”，故选A项。
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