$3.$ What is the primary goal of solvent selection in extraction processes?

$-$ A$.$ Maximize cost

$-$ B$.$ Maximize reaction speed

$-$ C$.$ Maximize selectivity and efficiency

$-$ D$.$ Maximize pressure

$4.$ What defines a single$-$stage extraction process?

$-$ A$.$ Multiple contacts between phases

$-$ B$.$ One$-$time contact between phases

$-$ C$.$ Continuous contact between phases

$-$ D$.$ No contact between phases

$5.$ Which of the following is a characteristic of multistage cross$-$current extraction?

$-$ A$.$ Solvent is reused in each stage

$-$ B$.$ Each stage has fresh solvent

$-$ C$.$ Only one stage is used

$-$ D$.$ Solvent and feed are in the same phase

$6.$ In co$-$current extraction, how do the phases move?

$-$ A$.$ In opposite directions

$-$ B$.$ In the same direction

$-$ C$.$ In perpendicular directions

$-$ D$.$ Randomly