A scientist who works in animal physiology wants to study perception of uv light in rats. She randomly selects 20 rats and records how far each rat travels in 5 minutes inside a dark maze. The distance is recorded and then the rats are returned to the starting position. The maze is then "illuminated" with uv light and the rats are again given 5 minutes to travel through the maze. This distance is then recorded. Which hypothesis test would be most appropriate for this study?.