The resource use efficiency hypothesis is an explanation for why productivity increases when plant species diversity increases. Which of these predictions follows from this hypothesis?

a) There should be a strong positive relationship between the number of functional groups in an experimental plot and biomass.

b) If a certain big-producer is present, then biomass in a plot is high. If the big producer is absent, biomass is low.

c) Productivity should be the highest in the tropics, where species diversity is the highest.

d) The more mutualists are present (e.g. beneficial fungi, nitrogen-fixing plants), the higher the biomass.