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.