在大量的实验中，某一事件发生的几率等于它不发生的几率。

　　24. Most substance contract when they freeze so that the density of a substance’s solid is higher than the density of its liquid.

　　大多数物质遇冷收缩，所以他们的密度在固态时高于液态。

　　25. The mechanism by which brain cells store memories is not clearly understood.

　　大脑细胞储存记忆的机理并不为人明白。

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