What is a critical load?

What is a critical load?

A) the level of a given toxin in an ecosystem that is lethal to 50% of the species present
B) the number of predators an ecosystem can support that effectively culls prey populations to healthy levels
C) the amount of nutrient augmentation necessary to bring a depleted habitat back to its former level
D) the amount of added nutrient that can be absorbed by plants without damaging ecosystem integrity
E) the maximum abundance level of a particular species, beyond which additional numbers will degrade a habitat