You are presented with the question "Some aviary species fly and others swim but, all endothermic tetrapod vertebrates share what trait?" and the following answer choices: 
- warm-blooded
 - Weight
 - fur and fat
 - sweating
 - Epidermis
 - feathers
 - retina
 - four limbs

Now knowing that birds are endothermic tetrapod vertebrates and some birds fly and some birds swim, but all birds have feathers, choose the best answer.


 

      feathers   