Birds have relatively high metabolic rates, and feathers act to insulate their bodies. These observations support which conclusion about birds?
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A. Birds are ectotherms that take on the temperature of their surroundings.
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B. Birds are endotherms that conserve body heat to maintain a constant body temperature,
C. Birds are ectotherms that must constantly release heat to avoid overheating,
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D. Birds are endotherms that must bask in the sun to keep their bodies warm.