You are presented with the question "what are amphibians classified by?" and the following answer choices: 
- organ
 - ovaries
 - colors
 - feet
 - alleles
 - genus
 - skull
 - Length

Now knowing that amphibians are the first true tetrapods, or vertebrates with four limbs and modern classifications of the tetrapods rely on the structure of the skull , choose the best answer.


 

       skull  