The correct answer is C) the cylinders are not chilled by the expansion of the starting air. In a medium-speed marine propulsion engine equipped with direct admission air starting valves, the cylinders without air starting valves fire first because the expansion of the starting air in the cylinders with air starting valves cools those cylinders, reducing the initial compression temperature and making it more difficult for those cylinders to fire on the first revolution. The cylinders without air starting valves are not subjected to this cooling effect, allowing them to fire first and initiate the engine's operation. The other answer choices are incorrect because: A) the fuel is admitted to all cylinders during cranking, not just the ones without air starting valves; B) the operation is not necessarily under higher compression in the cylinders without air starting valves; and D) the compression is not released by opening the exhaust valves during starting, as this would prevent the engine from starting.