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mountain a large natural elevation of the earth's surface rising abruptly from the surrounding level; a large steep hill.
plain is a flat region. Plains occur as lowlands and at the bottoms of valleys but also on plateaus or uplands at highelevations.

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A mountain is a raised land which is very high. A characteristic of mountains are - 
1) They have a sharp tip known as a peak.
3) Some examples of mountains are Himalaya in India and Mt. Everest in Nepal.
A plain is a flat land. A characteristic of plains are - 
1) Land and soil in the plains are very fertile.
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