Read the following paragraph and choose the best topic sentence from the four options that follow.Write the corresponding letter of your choice on the answer sheet.
Zebras evolved from horses more than 2 million years ago, biologists have found. Scientists have previously assumed that zebras' stripes evolved for one, or a combination of four main reasons: confusing enemies, protecting against disease-carrying insects, controlling body temperature, and maintaining social unity. While numerous previous studies of the phenomenon focused on one single assumption, a study led by Brenda Larison, a researcher in UCLA, was the first to fully test a large set of assumptions against one another. The researchers examined zebras at 16 locations in Africa. On zebras in warmer climates, the stripes are bold and cover the entire body. On others-particularly those in regions with colder winters such as South Africa and Namibia-the stripes are fewer in number and are lighter and narrower. In some cases, the logs or other body parts have virtually no striping.