Scientist: Laboratory animals have access to ample food, and they get relatively little exercise. These factors can skew the results of research using animals, since such studies often rely on the assumption that the animal subjects are healthy. For instance, animal studies that purport to show that extreme caloric restriction can extend lifespans take for granted that their subjects were not overfed to begin with. The scientist's argument requires assuming which one of the following? ['Laboratory animals are healthy if they are fed a carefully restricted diet and get plenty of exercise.', 'Laboratory conditions that provide animals with ample food but relatively little exercise can be unhealthy for the animals.', 'Some animal studies take into consideration the differences between the living conditions of laboratory animals and those of other animals.', 'When provided with unlimited food over a long period of time, animals show little day-to-day variation in their eating habits.']