In most ecosystems, gross primary productivity-the total amount of chemical energy that producers create in a given time-is not entirely available for the consumers to utilize due to which explanation below?

(A) Not all solar energy is in the correct spectrum for plants to absorb it.
(B) Plants utilize some energy for the cellular respiration.
(C) Heterotrophs do not absorb energy from autotrophs.
(D) Very little biomass is available at the producer level.