plains are thickly populated due to  fertility in soil and agriculture is very much done here
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Plains are very thickly populated because
1.Plains have fertile alluvial soil which is suitable for growing all kinds of crops.So this area is good for agriculture
2.Raw materials are easily available ,so a large number of industries ,like agro based industries can be established.
3.Since industries provide employments so a large number of work seekers migrate to this area.
4.Due to flat land Transportation and communications system can be easily developed. This further boosts the population.