At a minimum, primary producers require water, carbon dioxide, and light. These are things that are readily available in most ecosystems. However, we do not observe the same rates of net primary productivity in every region of the globe. Which of the following statements BEST reflects one reason why rates of productivity are not the same everywhere?

A.) Rates of net primary productivity are actually the same everywhere; scientists just need to calculate the rates relative to plant biomass in a given ecosystem.
B.) Although water is limited in certain ecosystems, nutrients that are scarce actually limit net primary productivity because they are necessary for growth.
C.) Rates of net primary productivity are actually limited by the amount of solar radiation. If net primary productivity were calculated as a yearly average, then this would show that ecosystems have the same rate of productivity.
D.) Although water is limited in certain ecosystems, irrigation would solve the problem and equalize rates of net primary productivity in every ecosystem.