填空题 Wastes only become pollutants when their levels rise to the point at which nature"s 1 systems are overwhelmed and can no longer cope. (pure)
【正确答案】
【答案解析】purification[解析] 句意：只有当废物的量超过了一定的限度以至于大自然的自净系统不能正常运作时，废物才会变成污染物。这里是一个名词短语，因此要用pure的名词形式。purification system表示“自净系统”。)