The correct answer is d. An article on heart health written by a cardiologist. This question is asking you to identify credibility. You must ask yourself, is this author someone I can trust? Option a is incorrect, because there is no credibility in an advertisement. You know that the goal is to get you to buy something, which doesn't mean you're receiving the most accurate information. Option b is incorrect, because why would a Hollywood movie star have the credentials to discuss greenhouse effect? Finally, option c is incorrect because again, what gives this citizen the credentials to discuss the dangerous conditions. A cardiologist, on the other hand, specializes in hearts, so you know you can trust them.