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Câu hỏi :

Any change in one part of an ecosystem can cause changes in other parts. Droughts, storms and fires can change ecosystems. Some changes harm ecosystems. If there is too (1)____ rainfall, plants will not have enough water to live. If a kind of plant dies off, the animals that feed on it may also die or move away. Some changes are good for ecosystems. Some pine forests need fire for the pine trees to reproduce. The seeds are sealed inside pinecones. Heat from a forest fire melts the seal and lets the seeds (2)_____. Polluting the air, soil, and water can harm ecosystems. Building (3)_______ on rivers for electric power and irrigation can harm ecosystems around the rivers. Bulldozing wetlands and cutting down (4)________ destroy ecosystems. Ecologists are working with companies and governments to find better ways of catching fish, cutting down trees, and building dams. They are looking for ways to get food, lumber, and other products for people (5)________ causing harm to ecosystems