Two masses $m_1$ and $m_2$ are connected by a massless spring of spring constant $k$ and unstretched length $l$. The masses are placed on a frictionless straight channel,which we consider our $X$-axis. They are initially at rest at $x=0$ and $x=l$,respectively. At $t=0$,a velocity of $v_0$ is suddenly imparted to the first particle. At a later time $t$,the centre of mass of the two masses is at

• A
  $x=\frac{m_2 l}{m_1+m_2}$
• B
  $x=\frac{m_1 l}{m_1+m_2}+\frac{m_1 v_0 t}{m_1+m_2}$
• C
  $x=\frac{m_2 l}{m_1+m_2}+\frac{m_2 v_0 t}{m_1+m_2}$
• D
  $x=\frac{m_2 l}{m_1+m_2}+\frac{m_1 v_0 t}{m_1+m_2}$