$10.$ In counter$-$current operations, the $\_\_\_$ of one phase is in direct contact with the $\_$ of the other phase, maximizing the $\_\_\_$ driving force for mass transfer.

$-$ a$)$ inlet, outlet, overall

$-$ b$)$ outlet, inlet, partial

$-$ c$)$ midsection, midsection, local

$-$ d$)$ top, bottom, dynamic