That means keeping the land, water and air clean.

这意味着要保持土地、水和空气的清洁。

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He swims to teach people about the importance of keeping water clean.

他游泳是为了教导人们保持水清洁的重要性。

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Then the researchers started keeping the animals in clean cages. Really clean cages, in which the bedding, food, and water are sterilized.

研究人员将它们放入洁净的笼中，这可是真正的一尘不染——它们的寝具、食物、饮水全都是经过消毒无菌的。

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The region has the highest water bills in the UK, partly because of the cost of keeping the coastal region's sea water clean.

BBC: Water customers have not been given a rebate since 1998

Much play was made a few years ago of the role played by woods and wetlands in upstate New York in keeping New York City's water supply clean.

ECONOMIST: The World Parks Congress