A salesman for a shoe company claimed runners would run faster races if they wore the company's brand of running shoe. A track coach wanted to see how much faster his runners would run, if they wore the company's shoes. He randomly selected 8 runners and had each of them run the 100-yard dash, twice. They wore their usual shoes to run the race on one day, and they wore the company's shoe to run the race on another day. Their times for both races were recorded, and the coach randomly selected the day each runner would wear the company's brand of shoe. Which confidence interval would be most appropriate for this study