You are presented with the question "What mammals have a high energy requirement and generate heat?" and the following answer choices: 
- cats
 - bats
 - Crustaceans
 - bears
 - frogs
 - lizards
 - birds
 - mammals

Now knowing that mammals generate heat mainly by keeping their metabolic rate high and birds have a high metabolic rate and a high energy requirement, choose the best answer.


 

       birds  