单选题
Why is it not necessary to provide powered flight in space all the time? ______
A、
Few spacecrafts complete their flight operations.
B、
The Earth's gravity cannot be overcome.
C、
No further power is needed after the escape speed is reached.
D、
"Free fall" takes the place of the Earth's gravity pull.
【正确答案】
C
【答案解析】
从文章第三段第一句“一旦火箭达到了脱离地心引力的速度，就不再需要更多的能量”，可以判断出此题的答案应为C。
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