Acanthostega is an extinct amphibian. Fossils indicate that it had four limbs, lungs, gills, and a tail used to propel itself through water. Why are Acanthostega fossils considered transitional fossils?
A. They display evidence of a form between an ancient fish and a modern bird.
B. They display evidence of a tetrapod with lungs.
C. They display evidence of gills and a fin-like tail.
D. They display evidence of a intermediate form between a fish and a tetrapod land animal.
