Researchers gave 100 first-graders after-school lessons in handwriting. They found that those whose composition skills had improved the most had learned to write letters the most automatically. This suggests that producing characters more automatically frees up mental resources for other activities. Which one of the following, if true, most strengthens the argument? ['Over the course of the lessons, the first-graders who showed greater improvement in their ability to write letters automatically also generally showed greater improvement in their composition skills.', 'The first-graders who wrote letters the most automatically before receiving the after-school lessons in handwriting showed the greatest improvement in their composition skills over the course of the lessons.', 'Among the first-graders who received the lessons in handwriting, those who started out with strong composition skills showed substantial improvement in how automatically they could write letters.', 'Before receiving the after-school lessons in handwriting, the 100 first-graders who received the lessons were representative of first-graders more generally, with respect to their skills in both handwriting and composition.']