D. The higher temperatures found in urban heat islands are concerning for a number of reasons.

Question: "(16) First, excessive heat can cause physical problems such as dehydration for people and some animals. (17) Plus, muggy heat is uncomfortable—a fact that leads many city dwellers to run their air conditioners around the clock. (18) Not only does this place enormous demands on the energy supply, it can also be very expensive." Which sentence should come before sentence 16 to serve as a topic sentence for the fourth paragraph (sentences 16-18)? A.Urban heat islands can be located by measuring the temperatures in the city and surrounding areas. B.Anyone who has spent time in an urban heat island knows that the stifling heat can be unbearable. C.Research centers are beginning to pay closer attention to areas designated as urban heat islands. D.The higher temperatures found in urban heat islands are concerning for a number of reasons.