42.ThefreemovementofpeopleandgoodsisacornerstoneoftheEuropeanUnion,referredtobytheshorthand“Schengen,”afterthecityinLuxembourgwherethe1985treatythatcreatedwhatisnowa26-nation,passport-freezonewassigned.VocabularyBuildingwhichofthefollowingcanbestreplace"cornerstone"in"thefreemovementofpeopleandgoodsisacornerstoneoftheEuropeanUnion?