单项选择题

When you look at the sky on a clear day, it seems as if the Earth is covered by a blue dome (圆盖). The sky is blue (41) what happens to sunlight as it enters the Earth’s atmosphere. Sunlight (42) light rays of all lengths. The longer rays make up red light and the shorter rays make up blue light. (43) sunlight enters the Earth’s atmosphere, it strikes the tiny (44) of air and is scattered. The scattered light is made up mostly of violet, blue, a little green, and a little yellow and red. The (45) of these colors is sky blue.
Sometimes (46) sunset the sky looks red. The sunlight is coming toward you from a lower (47) in the sky. The lower the sun, the (48) is the path that the light rays must travel through the air. The shorter blue rays (49) immediately when striking molecules of air. The sunlight that you see (50) the blue and violet rays that the air has scattered. As the sun gets (51) in the sky, the light rays must pass through more air, and so the longer orange and red rays also become scattered. At sunset, the shorter blue rays never get through the atmosphere, (52) you do not see them.
If you stand at a place (53) the surface of the Earth is flat, the sky seems to meet the Earth (54) any direction you look. The point at which the sky and Earth (55) to "meet" is called the horizon. The horizon is (56) the same distance from you, no matter in what direction you look. If you walk toward the horizon, you do not get (57) to it. No matter (58) you are, the surface of the Earth (59) downward and away from you. The distance at which you can no longer see the Earth’s surface (60) it has curved down too far—is the horizon.

57（）

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