The U - boat could keep the sea for three months.

(德国)潜艇能不靠岸在海上航行3个月.

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The answer is that seaweeds reduce various chemicals that help keep the sea water clean.

答案是海藻产生出各种各样化学物质,可能帮助使海水保持清洁.

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There are many dykes everywhere which keep the sea away from the land.

到处有堤防止海水侵入陆地.

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The answer is that seaweeds produce various chemicals that help keep the sea water clean.

答案是海藻产生出各种各样的化学物质,这些化学物质有助于使海水保持清洁.

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