Birds and humans are theorized to have a common ancestor because they have homologous structures. which evidence best supports this theory? they both have tails as embryos. they each have vestigial structures. they have analogous structures. they have different forms as adults.

The evidence that best supports this theory is:

They both have tails as embryos.

Darwin created a theory about homologous structures when he was studying anatomy. A homologous structure is a structure that is similar to different types of animals. He theorized that the more homologous structure two species of animals share, the closer back their common ancestor was (are related closely).
Moreover, Darwin’s notion that human tails are mere vestiges of our ancestry have also promoted the idea that these tails are benign structures that can be removed with little thought or concern about other potential problems.

The answer is a because it say when u add the h+ ions in the periodic table