Journalist: Obviously, though some animals are purely carnivorous, none would survive without plants. But the dependence is mutual. Many plant species would never have come to be had there been no animals to pollinate, fertilize, and broadcast their seeds. Also, plants' photosynthetic activity would deplete the carbon dioxide in Earth' s atmosphere were it not constantly being replenished by the exhalation of animals, engine fumes, and smoke from fires, many set by human beings. Which one of the following most accurately expresses the main conclusion of the journalist's argument? ['The photosynthetic activity of plants is necessary for animal life, but animal life is also necessary for the occurrence of photosynthesis in plants.', 'Some purely carnivorous animals would not survive without plants.', "Human activity is part of what prevents plants from depleting the oxygen in Earth's atmosphere on which plants and animals alike depend.", 'Just as animals are dependent on plants for their survival, plants are dependent on animals for theirs.']