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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单项选择题
What is the most important factor for a rocket to escape from the Earth ______
A．To travel as fast as it can.
B．To overcome the Earth gravity.
C．To get away from the air resistance.
D．To have strong power in its operation.
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.
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