A seismic refraction test was conducted on a multi$-$layered soil deposit. The velocities of the $1$st and $2$nd layers were found to be $600$ m$/$s and $1550$ m$/$s respectively. Determine the critical distance $($xc$)$ of the geophone. Given t$0=10-2$ s$.$

a$.9\mathrm{.}77$ m

b$.16\mathrm{.}0$ m

c$.3\mathrm{.}06$ m

d$.12\mathrm{.}2$ m