随着原油储量的递减及原油的深度开采，世界性原油质量逐渐变重、变劣，轻质油品含量降低，必须进行重油轻质化。
Due to the reduction of petroleum reservoir and deeper extraction, the crude oil is heavier and poorer in the world, and the light products in the oil is becoming decreased.
云中的小水滴不断地相互碰撞，逐渐变、变重，最后变成雨落到地面。
When the droplets in the clouds collide, they become too big and heavy, they fall to earth again as rain.
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