THIS IS THE ANSWER TO A QUESTION!! Which describes a difference between birds and mammals?

Birds protect and feed their young.
Birds have nearly hollow bones.
Birds undergo internal fertilization.
Birds efficiently transport oxygen to cells.

the answer is B, birds have nearly hollow bones.

explanation, if you want one to help clarify:
birds have hollow bones which can aid whenever they flying, they are light so its easier to glide through air using their wings, if they had bones like us humans i doubt they'd be able to fly using their wings as they do now.