$9.$ The Fenske equation primarily estimates:

$-$ A$)$ The minimum number of theoretical stages at total reflux.

$-$ B$)$ The optimum feed stage location.

$-$ C$)$ The maximum possible recovery of a distillate component.

$-$ D$)$ The heat duty required for reboiler.

$10.$ The Underwood equations are used to calculate:

$-$ A$)$ The minimum reflux ratio for a given separation.

$-$ B$)$ The total number of stages in a distillation column.

$-$ C$)$ The optimum reflux ratio.

$-$ D$)$ The heat exchange efficiency.

$11.$ Differential distillation is characterized by:

$-$ A$)$ A constant composition of the distillate.

$-$ B$)$ A changing composition of the distillate over time.

$-$ C$)$ No change in the boiler composition.

$-$ D$)$ The use of a packed column.

$12.$ In flash distillation, the feed is:

$-$ A$)$ Heated to its boiling point, then all vaporized at once.

$-$ B$)$ Partially vaporized in a single equilibrium stage.

$-$ C$)$ Completely condensed before distillation.

$-$ D$)$ Passed through multiple distillation stages.