Doctor: Medical researchers recently examined a large group of individuals who said that they had never experienced serious back pain. Half of the members of the group turned out to have bulging or slipped disks in their spines, conditions often blamed for serious back pain. Since these individuals with bulging or slipped disks evidently felt no pain from them, these conditions could not lead to serious back pain in people who do experience such pain. The reasoning in the doctor's argument is most vulnerable to the criticism that it fails to consider which one of the following possibilities? ['A characteristic found in half of a given sample of the population might not occur in half of the entire population.', 'A factor that is not in itself sufficient to produce a certain effect may nonetheless be partly responsible for that effect in some instances.', 'A factor that need not be present in order for a certain effect to arise may nonetheless be sufficient to produce that effect.', 'An effect that occurs in the absence of a particular phenomenon might not occur when that phenomenon is present.']