According to embryology, why do fish and birds have a common ancestor?

They both have embryos with gill slits and tails.
They both have embryos with traits that eventually disappear.
They both have embryos that develop into adults.
They both have embryos that form from a fertilized egg.

•there must be variation for the particular trait within a population.

•the variation must be inheritable (that is, it must be capable of being passed on from the parents to their offspring).

•individuals with one version of the trait must produce more offspring than those with a different version of the trait.good

explanation:

i think the answer would be d