【单选题】

Because we can feel that things are heavy, we think of weight as being a fixed quality in an object, but it is not really fixed at all. If you could take a onepound packet of butter 4, 000 miles out from the earth, it would weigh only a quarter of a pound. 

Why would things weigh only a quarter as much as they do at the surface of the earth if we took them 4 000 miles out into space? The reson is this: All objects have a natural attraction for all other objects; this is called gravitational attraction, but this power of attraction between two objects gets weaker as they get farther apart. When the butter was at the surface of the earth, it was 4 000 miles from the centre (in other words the radius(半径)of the earth is 4 000 miles). When we took the butter 4 000 miles out, it was 8 000 miles from the centre, which is twice the distance. 
If you double the distance between two objects, their gravitational attraction decreases(减少)two times two. If you treble(翻三倍)the distance, it gets nine times weaker(three times three). If you take it four times as far away, it gets sixteen times weaker(four times four)and so on. 
69. The best title for this passage is. 
A、The Earth Weight 
B、Weight in Space 
C、Changing Weight on the Earth 
D、Weight on and off the Earth 
70. We can feel things are heavy because. 
A、weight is a fixed quality in an object 
B、they are far away from the centre of the earth 
C、of the earth's strong attraction for them 
D、they are not taken away from the surface of the earth 
71. 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 
72. If an object weighed one pound 8 000 miles above the earth, it would weighon the surface of the earth. 
A、6 pounds 
B、4 pounds 
C、9 pounds 
D、1/9 pound