【正确答案】 A

【答案解析】[听力原文]
We are so used to our life on the surface on the Earth that it's not easy for our minds to break free of all the ideas that we take for granted. For example, we can feel things are heavy so we think of weight as a fixed quality in an object. But actually, it is not 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. The reason for such change of weight is this: all objects have a natural attraction for all other objects. This is called the gravitational attraction. But this power of attraction between two objects gets weaker as they get farther apart. So the weight of an object on the surface of the Earth is not the same as its weight 4,000 miles out in space. What about the weight of one-pound of butter on the surface of the moon? There will be attraction between the butter and the moon, but the butter will weigh only about one-sixth as much as it does on Earth. This is because the moon is so much smaller than the Earth. The amount of the gravitational pull that a body produces depends on the amount of material in it. A packet of butter has a gravitation pull of its own, but it is very small in relation to the pull of something as large as the moon, or the Earth.

According to the speaker, what quality of an object is taken for granted by people?