Distance between two parametric curves

Question

Given the trajectories of two objects $$r(t)=(4\cos(t),2\sin(t))$$ $$s(t)=(2\sin(2t),3\cos(2t))$$ I have to evaluate $$\left.\frac{dD}{dt}\right|_{t=\pi}$$ where $D$ is the distance between them. May I say $$D(t)=r(t)-s(t)?$$ Or should it be $$D(t)=\sqrt{r^2(t)-s^2(t)}?$$