The term food chain was first proposed by British zoologist Elton in 1927. If a toxic substance is absorbed by the lower part of the food chain, such as by grass, although the concentration is low, it does not affect the growth of the grass, but the rabbit eats grass Later toxic substances are difficult to excrete and will accumulate in its body; eagles eat a large number of rabbits, and toxic substances will further accumulate in the eagle. The reason why the American national bird Bald Eagle is facing extinction is not because of being killed but by harmful chemicals DDT gradually accumulates in the body, resulting in eggs that are soft-shelled and unable to hatch. What does this text introduce? ['Origin and meaning of the term food chain.', 'Causes of the Extinction of the American Bald Eagle.', 'Cumulative and magnifying effects of the food chain.', 'The environment has a very important impact on the food chain.']