[单项选择] We all admire the beautiful blue of a clear sky or the superb contrast between the brilliant white of the clouds and the skyblue between them. This colour, however, is not natural in the atmosphere; it is an optical effect(光学效应). When light encounters a cloud of small particles, it is diffused. This may be illustrated by a familiar example. When sunlight enters a darkened room through a gap in the blinds, it will appear as a brilliant ray of light. This is because there are tiny particles of dust suspended in the air. These, although normally invisible, catch and diffuse any strong light shining on them. The same effect can be seen in the sky when broken clouds hide the sun. Thus we can see that diffusion occurs owing to all sorts of impurities in the atmosphere. This explains all the delightfully varied shades of colour seen at different times of the day. White light is a mixture of all the colours of the rainbow or spectrum (光谱) , which, as Isaac Newton proved, can be s
A. near a mountain
B. against a mountain
C. from the top of a mountain
D. at the foot of a mountain

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