What is the basis of the researchers' new theory A: They performed mathematical calculations and determined that dinosaurs must have had four chambered hearts. B: They found a fossil of an entire dinosaur end reviewed the arteries and veins flowing from and to the heart. C: They found a fossil of a dinosaur's heart and discovered it had four chambers and one aorta. D: They viewed a fossil of a dinosaur's heart and discovered that it had two aortas.