Two small blocks, block 1 of mass \(m_1\) and block 2 of mass \(m_2\), are connected by a light rigid rod of length \(L\) and slide to the right with initial velocity \(v_0\) across a horizontal surface. The surface has a coefficient of kinetic friction \(\mu_k\) for \(x < 0\) and is frictionless for \(x \ge 0\). Block 1 crosses the boundary at \(x = 0\) at time \(t_1\), and block 2 crosses the boundary at time \(t_2\), with the system maintaining forward motion at all times. Which of the following best describes the shape of the graph of the center-of-mass velocity \(v_{\text{cm}}\) of the two-block system as a function of time \(t\)?