Why Are Grass-Fed Animals Considered to Represent a Net Gain

Why are grass-fed animals considered to represent a net gain for the human food supply?

A) They are raised following ethical guidelines.
B) They only eat what they have naturally evolved to eat-that is, grass.
C) They eat grass that humans cannot eat on land that is otherwise unsuitable for human crops.
D) They are raised on pasture land that does not require pesticides and fertilizer.
E) All of these answers are correct.