with the development of civilization man's interference in nature began to increase.Large cities with thousands of smoky industrial enterprises appear all over the world today. The pollution of air and the world's ocean, destruction of the ozone layer is the result of man's careless interaction with nature, a sign of the ecological crises.Environmental protection is of a universal concern. As many as 159 countries — members of the UNO — have set up environmental protection agencies. The international organisation Greenpeace is also doing much to preserve the environment.
present
i swim
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i am swimming
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simple past
i swam
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past continuous
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