ANS:C - both parallel and straight

The assumption made in the Ellis method in distillation is that enthalpy concentration lines of vapor and liquid are both parallel and straight. In the Ellis method, which is used for the design and analysis of distillation columns, the enthalpy concentration lines for both vapor and liquid phases are assumed to be parallel and straight. This assumption simplifies the calculations involved in determining the number of theoretical stages required for separation. Parallel lines imply that the slopes of the enthalpy concentration lines for both vapor and liquid phases are equal, indicating that the relative volatility of the components does not change significantly with changes in composition or temperature along the column. Straight lines indicate that the enthalpy concentration lines are linear, simplifying the calculation of the equilibrium relationship between the vapor and liquid phases at each stage. While these assumptions may not hold true in all cases, they provide a useful starting point for preliminary design calculations in distillation processes.