Two electric charges are separated by a certain distance. The first charge is $+3\backslash$mu C and the second charge is $+9\backslash$mu C$.$ The magnitude of the electric force the first charge exerts on the second charge is $2\mathrm{.}0$ mN$.$ How large is the magnitude of the electric force the second charge exerts on the first charge? $($in mN$)$

A: $0\mathrm{.}22$ B: $0\mathrm{.}67$ C: $2\mathrm{.}0$ D: $6\mathrm{.}0$ E: $18$