Essayist: Common sense, which is always progressing, is nothing but a collection of theories that have been tested over time and found useful. When alternative theories that prove even more useful are developed, they gradually take the place of theories already embodied in common sense. This causes common sense to progress, but, because it absorbs new theories slowly, it always contains some obsolete theories. If all of the essayist's statements are true, then which one of the following must be true? ['Of the useful theories within the body of common sense, the older ones are generally less useful than the newer ones.', 'Each theory within the body of common sense is eventually replaced with a new theory that is more useful.', 'At least some new theories that have not yet been found to be more useful than any theory currently part of common sense will never be absorbed into the body of common sense.', 'At least some theories that have been tested over time and found useful are less useful than some other theories that have not been fully absorbed into the body of common sense.']