单项选择题

The main idea of this passage is

A．different weight systems in different places.
B．freedom from traditional ideas.
C．traveling from the earth to the moon.
D．the effect of gravitation on weight.
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单项选择题
According to the passage, weight should be understood in the sense that
A．it is fixed if it is outside the earth's gravitational pull.
B．it decreases four times when it is 4,000 miles from the earth's center.
C．it varies with the change of the gravitational attraction between two objects.
D．things increase in amount as they are closer to the earth's surface.
单项选择题
We find it hard to break free from ideas connected with living on the earth because
A．it demands too great an effort for us to do so.
B．we are too familiar with the way things are to question the ideas.
C．we have proved that those ideas are correct.
D．we are so sure of ourselves that we never doubt anything on the earth.
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