Why Is It More Energetically Efficient to Be a Vegetarian

Why is it more energetically efficient to be a vegetarian than to eat meat?

A) Plants contain more energy than meat since they are at the lowest trophic level.
B) A person burns less energy digesting plants than digesting meat.
C) It takes less effort to gather plants to eat than to prepare and cook meat.
D) No energy is lost to another animal's metabolism when a person eats plants.
E) It is easier to extract energy from plants than from animal sources.