$9.$ The overall mass transfer coefficient $($K$)$ is related to the individual mass transfer coefficients by:

$-$ a$)1/$K $=1/$k$\_$L $+1/$k$\_$G

$-$ b$)1/$K $=1/($k$\_$L $*$ k$\_$G$)$

$-$ c$)$ K $=$ k$\_$L $*$ k$\_$G

$-$ d$)$ None of the above