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　　D 4 pounds.
　　42 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.
　　43 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.
　　44 We do not feel the gravitational pull of a packet of butter because___________
　　A it is too small to have a gravitational pull of its own.
　　B its pull is so small that we tend to ignore it.
　　C its pull disappears in the presence of the earth's gravitation.
　　D it tends to melt and loses its gravitational pull.
　　45 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.