[A] : The heart of birds is more advanced than the four chambered heart of crocodiles.
[R]: in the bird s heart, there is a complete elimination of left aorta.
A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false