$18.$ In extraction, the term "selectivity" refers to:

$-$ A$.$ The solvent's ability to dissolve all components equally

$-$ B$.$ The solvent's preference for one component over others

$-$ C$.$ The speed at which the solvent dissolves the solute

$-$ D$.$ The temperature at which the solvent works best

$19.$ The main disadvantage of using highly selective solvents is:

$-$ A$.$ High cost

$-$ B$.$ Low extraction efficiency

$-$ C$.$ High toxicity

$-$ D$.$ Difficulty in solvent recovery

$20.$ Counter$-$current extraction is most efficient when:

$-$ A$.$ A high degree of separation is not required

$-$ B$.$ Only a small amount of extract is needed

$-$ C$.$ A high degree of separation is required

$-$ D$.$ The solvents are miscible