Preschool children who spend the day in daycare nurseries are ill more often than those who do not. They catch many common illnesses, to which they are exposed by other children in the nurseries. However, when these children reach school age they tend to be ill less often than their classmates who did not spend the day in daycare nurseries during their preschool years. Which one of the following, if true, best explains the discrepancy in the information above? ['The number of infectious illnesses that children in a daycare nursery or school develop is roughly proportional to the number of children in the facility, and daycare nurseries are smaller than most schools.', 'Although in general the illnesses that children contract through contact with other children at daycare nurseries are not serious, some of those illnesses if untreated have serious complications.', 'Those children who have older siblings are likely to catch any common infectious illnesses that their older siblings have.', 'By school age, children who have been in daycare nurseries have developed the immunities to common childhood illnesses that children who have not been in such nurseries have yet to develop.']