ANS:C - both (a) & (b)

In the context of distillation, "straight" and "parallel" refer to the lines plotted on the McCabe-Thiele diagram, which is a graphical method used to analyze binary distillation processes.

1. Straight Lines: Both the equilibrium line and the operating line on the McCabe-Thiele diagram are straight. The equilibrium line represents the equilibrium relationship between the vapor and liquid phases at each stage of the distillation column. It is a straight line because it depicts the equilibrium between the vapor and liquid compositions for a given temperature and pressure.
2. Parallel Lines: In addition to being straight, the equilibrium line and the operating line are parallel to each other on the McCabe-Thiele diagram. This parallelism indicates that the slope of the operating line is equal to the slope of the equilibrium line. The operating line represents the relationship between the compositions of the vapor and liquid phases at each stage of the distillation column under actual operating conditions. When the overall efficiency and Murphree plate efficiency are equal, it implies that both lines have the same slope, resulting in parallelism.