[A] : A complete double circulation occurs in birds for the first time.
[R]: At no point is there any opportunity for oxygenated and unoxygenated blood to mix.
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