矿山地压是矿山生产活动中一种常见的自然现象。

Mine pressure is a kind of common natural phenomena in mine production.

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云是一种常见、普遍的自然现象，云的建模和渲染是计算机图形学领域的一个难点问题。

Cloud is a common, widespread natural phenomenon, cloud modeling and rending are a difficult problem in the field of computer graphics.

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