模态逻辑是关于必要性和可能性的逻辑。

Modal logic is the logic of necessity and possibility.

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但是，在必要性和可能性之间存在着一道鸿沟。

Between need and possibility, however, there is a wide gulf.

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从理论上论证了我国实施小班化教育的必要性和可能性。

It demonstrates in theory the necessity and possibility of having small class education in China.

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