Two particles of mass $m$ and $2m$ have their position vectors as a function of time as $r_1\left(t\right)=t \hat{\mathrm{i}}-t^3\widehat{ \mathrm{j}}+2t^2 \hat{\mathrm{k}}$ and $r_1\left(t\right)=t \hat{\mathrm{i}}-t^3 \hat{\mathrm{j}}-t^2 \hat{\mathrm{k}}$ respectively (where $t$ is the time). Which one of the following graphs represents the path of the centre of mass?