Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Which of the following structures does a frog develop as it changes from a tadpole to an adult frog?
lungs
Metamorphosis (frog develops limbs and lungs, transforms from aquatic to terrestrial).