单项选择题

When a bicycle "coasts" downhill, it ______.

A．runs along the hillside
B．does not need any further power
C．runs freely to all directions
D．does not meet with air resistance
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单项选择题
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.
单项选择题
How fast will a rocket go to escape completely from the Earth's gravity ______
A．Seventy miles per second.
B．Seven thousand miles per hour.
C．Twenty-five thousand miles per second.
D．Twenty-five thousand miles per hour.
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