Compressor fundamentals: select the correct statement among the following. Evaluate each statement about definitions and work in air compression, and choose the only correct one.

Concept/Approach
Check each statement against standard definitions and thermodynamic results for gas compression.

Analysis of options
A: The ratio p₂/p₁ is the compression ratio, not efficiency → false.B: For a compressor, discharge pressure p₂ > inlet pressure p₁, so p₂/p₁ > 1 → true.C: Work per cycle divided by stroke volume defines mean effective pressure, not capacity → false.D: For a given pressure ratio, isothermal compression requires the minimum work; polytropic/adiabatic require more → false.

Final Answer
The compression ratio for a compressor is always greater than unity.