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  3. 　　Every year millions of salmon swim from the ocean into the mouth of rivers，and then steadily up the rivers．Passing through wat

　　Every year millions of salmon swim from the ocean into the mouth of rivers，and then steadily up the rivers．Passing through wat

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答案　　Every year millions of salmon swim from the ocean into the mouth of rivers，and then steadily up the rivers．Passing through waters，around rocks and waterfalls，the fish finally reach their original streams or lakes．They dig out nests in the riverbed and lay their eggs．Then，exhausted by their journey，the parent salmon die．They have finished the task that nature has given them．Months or years later，the young fish start their trip to the ocean．They live in the salt water for two to seven years until they too are ready to swim back to reproduce．Their life cycle helps man provide himself with a basic food—fish．When the adult salmon gather at the river mouths for the annual trip up the rivers，they are in the best possible condition．And nearly every harbour has its salmon fishing fleet ready to catch thousands for markets．
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