[A]: At any given `CO_2` tension, reduced blood takes up larger amount of `CO_2` than oxygenated blood.
[R]: Oxygenation of blood causes evolution of `CO_2` in lungs.
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