虽然早期的潜艇并不适于航海，但它们直到现在取得了卓越的进步。

Although early submarines were less-than-seaworthy, their progression through present times has been remarkable.

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酷热的非洲，几乎没有适于航海的河流，这些河流从内陆高原到海岸平原一路穿瀑布、过险滩，倾流而下。

Few of tropical Africa's rivers are navigable from the ocean, dropping as they do from interior tableland to coastal plains by a series of falls and rapids.

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