单项选择题

If the distance between two objects is shortened by half, their gravitational attraction will______.

A．double
B．become four times stronger
C．be the same
D．get four times weaker
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单项选择题
We can feel things are heavy because ______.
A．weight is a fixed quality in an object
B．they are far away from the center of the earth
C．of the earth's strong attraction for them
D．they are not taken away from the surface of the earth
单项选择题
The way of communication ______ made use of were visible signs.
A．French
B．Roman
C．African
D．American
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