[单项选择] The United States In 1782 【51】 the United States won its independence, the bald eagle was chosen 【52】 the national bird of the new country. American leaders wanted the eagle to be a 【53】 of their country because it’’s a bird of strength and courage. They chose the bald eagle because it was found all over North America and only in North America. But a little over 200 years 【54】 ,the bald eagle had almost disappeared from the country, In 1972, there were only 3,000 bald eagles in the entire United States. The reason for the birds 【55】 population was pollution, especially pollution of rivers by pesticides. Pesticides are chemicals 【56】 to kill insects and other animals that attack and destroy crops. 【57】 , rain often washes pesticides into rivers. Pesticides pollute the rivers and poison the fish. Eagles eat 【58】 and the poison 【59】 their eggs. The eggs have very thin shells and do not hatch. Eagles 【60】 only two or three eggs a year. Beca
A. symbol
B. mark
C. trace
D. sign

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