$5.$ The material balance equation for a counter$-$current extraction process involves the $\_\_\_$ of the solute in the $\_$ phase, the $\_\_\_$ rate of the carrier, and the distribution coefficient.

$-$ a$)$ concentration, raffinate, flow

$-$ b$)$ mass, extract, transfer

$-$ c$)$ pressure, continuous, reaction

$-$ d$)$ temperature, dispersed, separation