ANS:B - mole ratio

For a single-component absorber, the operating line is straight only when plotted in terms of partial pressure units. In absorption processes involving a single solute component, the operating line represents the relationship between the concentrations (or partial pressures) of the solute in the gas and liquid phases. The slope of the operating line is typically determined by the equilibrium relationship between the solute concentrations in the two phases. When plotted in terms of partial pressure units, the relationship between the gas-phase and liquid-phase concentrations of the solute is often linear, resulting in a straight operating line. This is because, in many cases, the solubility of the solute in the liquid phase is directly proportional to its partial pressure in the gas phase according to Henry's law. Therefore, the operating line for a single-component absorber is straight only when plotted in terms of partial pressure units.