Proposals for extending the United States school year to bring it more in line with its European and Japanese counterparts are often met with the objection that curtailing the schools' three-month summer vacation would violate an established United States tradition dating from the nineteenth century. However, this objection misses its mark. True, in the nineteenth century the majority of schools closed for three months every summer, but only because they were in rural areas where successful harvests depended on children' s labor. If any policy could be justified by those appeals to tradition, it would be the policy of determining the length of the school year according to the needs of the economy. Which one of the following principles, if accepted, would provide the strongest justification for the conclusion? ['The actual tradition embodied in a given practice can be accurately identified only by reference to the reasons that originally prompted that practice.', 'Traditional principles should be discarded when they no longer serve the needs of the economy.', 'Because appeals to tradition often serve to mask the real interests at issue, such appeals should be disregarded.', 'That a given social policy has traditionally been in force justifies maintaining that policy only if doing so does not conflict with more pressing social needs.']