Read the excerpt from "killer whales and great white sharks.”

both killer whales and sharks have only one real enemy—people. both animals are threatened by pollution caused by humans. they can die from poisons in the water and in the animals they eat. killer whales often live fifty to eighty years in the wild. but when they are captured and trained to be in shows, they die much earlier. sharks are hunted for their meat and fins, even though this is against the law. sharks also get caught by accident in fishing nets. scientists now warn that great white sharks are in danger of dying off. they have been named an endangered species.

which best describes the structure of this excerpt?

the age at which sharks and whales usually die is being contrasted.
the biggest dangers faced by sharks and whales are being compared.
the reasons that sharks and whales are trained are being contrasted.
the rate at which sharks and whales are captured is being compared.