Most of it—97% —is in the form of the salt waters of the oceans. The rest is fresh, but three quarters of this is in the form of ice at the Poles and on mountains, and cannot be used by living systems when melted.
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【正确答案】 1、正确答案：语义错误，应把when改为until。    
【答案解析】解析：根据上下文可知，冰直到化成水后才能被生态系统里的生物使用，所以用not…until来表达。