[A]: A portal system is a complex of veins which begins in capillaries and ends in capil- laries before the blood which courses through it is retruned to the heart.
[R]: All vertebrates have a hepatic portal sys-tem.
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