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
As there is always the Earth's gravitational pull, people ______.
A．accept it without questioning it
B．never notice its presence
C．hardly think about how to use it
D．often try to escape from it
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