$6.$ In a counter$-$current process, if the solvent flow rate is $1$ kg$/$s and the solute flow rate is $0\mathrm{.}2$ kg$/$s$,$ the ratio of solvent to solute flow rate is $\_\_\_\_\_.$

$-$ a$)2$

$-$ b$)5$

$-$ c$)10$

$-$ d$)1$