What type of fossil evidence may exist that would lead scientists to believe that dinosaurs were ancestors of birds? why might large dinosaurs have died out at the end of the mesozoic era, while smaller birds survived?

b. the carbon cycle ensures living things have a source of hydrogen

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.