$10.$ For a counter$-$current extraction process, if the inlet concentration of solute is $3$ kg$/$m$\backslash (^3\backslash )$ and the outlet concentration is $1$ kg$/$m$\backslash (^3\backslash ),$ the concentration change is $\_\_\_\_\_$ kg$/$m$\backslash (^3\backslash ).$

$-$ a$)1$

$-$ b$)2$

$-$ c$)3$

$-$ d$)0\mathrm{.}5$