When birds are facing living pressures such as habitat destruction, natural enemy threats, and environmental pollution, they secrete a hormone called cortisol. Therefore, birds that are often under living stress have thinner feathers. Which of the following, if true, would support the above argument? ['Cortisol levels in birds may change due to global warming.', 'Hormone levels in the body increase after some birds change habitat.', 'Cortisol affects bird feather growth and metabolism.', 'If the feathers of birds become sparse, they must be under long-term stress.']