What part of this line most clearly adds to the tension in the story?

In e. c. tolman's experiment, some rats were trained to run through mazes for standard food goals, while other rats were allowed to explore the mazes for 10 days without food goals or other rewards. later, when food rewards were placed in a box at the far end of the maze, the previously unrewarded rats reached the food box as quickly as the rewarded rats after only one or two trials. this experiment demonstrated that the rats had the ability to form of their surroundings.