Regarding an axial-flow compressor, which statement is true?

The statement that every pair of rotor and stator blades constitutes a pressure stage is accurate because in an axial-flow compressor, the rotor blades are designed to impart energy to the airflow, while the stator blades serve to convert this energy into increased pressure. Each interaction between a rotor blade and a stator blade essentially changes the pressure of the airflow, creating a distinct pressure stage. This design allows for the efficient compression of air, as multiple stages can be stacked to achieve higher pressure ratios across the compressor.

The other options do not accurately reflect the principles of axial-flow compressors. Each rotor blade does not function completely independently; rather, they work in conjunction with both other rotor blades and the accompanying stator blades to achieve the desired aerodynamic effects. The relationship between airflow and pressure stages is more nuanced than suggested, as airflow usually increases with additional stages rather than leading to a decrease in pressure stages. Lastly, while fewer stages can indeed lead to greater pressure differences in certain scenarios, this principle is generally not consistent across all designs and should not be broadly applied without considering specific mechanical designs and efficiencies.