After combustion, the expanding gases __________.

The correct answer highlights the role of expanding gases in powering the compressor turbine. During the combustion process in a turbojet or turbofan engine, the fuel and air mixture ignites, producing high-temperature, high-pressure gases. These expanding gases then flow through the turbine section of the engine.

As these hot gases expand, they exert pressure on the blades of the compressor turbine. This interaction converts the energy from the expanding gases into mechanical energy, which drives the compressor. The compressor is responsible for drawing in air and compressing it before it enters the combustion chamber. This cyclical process is vital for maintaining engine performance and efficiency, continually enabling the system to function.

The other options incorrectly imply different roles for the components in the propulsion cycle. The reduction gear box and the power turbine are not directly driven by the expanding gases from the combustion process in the same manner as the compressor turbine. The power turbine, while also connected to the overall cycle, operates independently of the compressor turbine and has a different function within the propulsion system.