How did the skeleton change during bird evolution?

"The reduction in a bird's skeletal weight is largely a result of hollow construction of the skeleton. The fusion or loss of individual bones reduces the number of movable joints in the bird's skeleton, making it rigid and better adapted as the internal framework of a flying machine."

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.

a symbiotic relationship with the mitochondria