The future survival of the bald eagle is still an important American ecological concern.
A、migrationC
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ThefirstimportantexpositionintheUnitedStateswasheldinPhiladelphiain1876.
Blue－greenalgaegrowabundantlyinsaltmarshes.
Thecontrolsofmostmodernairplanescanbeoperatedeithermanuallyorautomatically.
Someanimalspantandsweattospeedevaporationofbodymoistureandthuscoolthemselves.
Thewheelsofthefirstroadvehicleswerefashionedfromcrudestonedisks.
