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单项选择题

Have you ever wondered what the sky really is Long ago many thought it circled the earth, carrying the sun, moon, planets, and stars with it. Since then scientists have found that it is really just space. In this space the earth, moon, planets, sun, and other stars are traveling.
At times the sky changes its color. When the sun rises or sets, red, orange, yellow, or pink colors are seen. During a clear day the sky is very blue, and on a rainy day it is gray. At night it seems almost black except for the light from the moon and stars.
People of old times thought the stars made pictures in the night sky. They saw bears, lions, and many other things. Sailors at sea and shepherds in fields with their sheep knew these. They need only to look. They can go out and view the night sky.
No one knows how many stars are there in the sky. They don’t know how far the sky reaches out in all directions from the earth. Each new and larger telescope that is built helps scientists see farther out into space. More distant stars and planets are there to be discovered.
Most people like to go up into the sky. They travel in balloons and airplanes. People have gone as far as the moon. Someday they will visit distant planets and beyond.

It can be inferred from the first paragraph that long time ago many people did not know the sky is ______.
A. a circle
B. space
C. a star
D. a planet
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单项选择题

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