[单项选择]Someday, in 62 the outer planets of our solar system, humankind will want to do 63 send small probes that merely fly rapidly by them. In time, we will want 64 spacecraft that will go 65 orbit around these alien planets, land robots on their moons, and even return rock and soil sample back to Earth. 66 , we will want to send humans to their moons, on at least 67 which liquid water—the fundamental requirementfor life 68 we know it—is believed to be 69 .
For missions such as these, we will need rockets powered by a nuclear reaction 70 igniting chemicals. Chemical rockets have served us well. But the 71 low amount of energy that they can deliver for a given mass of fuel is a severe 72 when dispatching spacecraft 73 long distances. To reach the outer planets, for example, a chemically-powered space vehicle must 74 fuel by having a very small mass
A. conserve
B. consume
C. exploit
D. contain

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[单项选择]Someday, in 62 the outer planets of our solar system, humankind will want to do 63 send small probes that merely fly rapidly by them. In time, we will want 64 spacecraft that will go 65 orbit around these alien planets, land robots on their moons, and even return rock and soil sample back to Earth. 66 , we will want to send humans to their moons, on at least 67 which liquid water—the fundamental requirementfor life 68 we know it—is believed to be 69 .
For missions such as these, we will need rockets powered by a nuclear reaction 70 igniting chemicals. Chemical rockets have served us well. But the 71 low amount of energy that they can deliver for a given mass of fuel is a severe 72 when dispatching spacecraft 73 long distances. To reach the outer planets, for example, a chemically-powered space vehicle must 74 fuel by having a very small mass
A. researching
B. exploring
C. developing
D. identifying

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