(A) : Primary productivity depends on the plant species inhabiting a particular area.
(R) : The annual net primary productivity of our biosphere is about 170 billion tons of organic matter.
A. If both (A) and (R) are true but (R) is not the correct explanation of (A).
B. If both (A) and (R) are true and (R) is the correct explanation of (A).
C. If (A) is true but (R) is false.
D. If both (A) and (R) are false.