To better reflect the age range of its citizens, a city council passed an ordinance providing that no city employee could be hired or promoted unless that employee had reached the age of 55. A 25-year-old city mechanic in the lower pay classification had recently finished first on a promotional exam for the senior mechanic position, which would entitle him to a substantial increase in pay. His supervisor told him that, under instructions from the head of the city administrative office, a 56-year-old city mechanic who scored lower on the exam would be promoted to the position instead. If the mechanic brings an appropriate action in federal court to challenge the ordinance, which party would bear the burden of proof?