$4.$ In a co$-$current flow process, the driving force for mass transfer $\_\_\_$ along the length of the equipment, the outlet concentration of the phases is $\_,$ and the overall transfer rate is typically $\_\_\_$ than in counter$-$current flow.

$-$ a$)$ decreases, similar, lower

$-$ b$)$ increases, different, higher

$-$ c$)$ remains constant, identical, higher

$-$ d$)$ fluctuates, inversely related, lower