Two particles of masses $m_1$ and $m_2$ initially at rest start moving towards each other under their mutual force of attraction. The speed of the centre of mass at any time $t$,when they are at a distance $r$ apart,is

• A
  zero
• B
  $\left( {G\frac{{{m_1}{m_2}}}{{{r^2}}} \cdot \frac{1}{{{m_1}}}} \right)\,t$
• C
  $\left( {G\frac{{{m_1}{m_2}}}{{{r^2}}} \cdot \frac{1}{{{m_2}}}} \right)\,t$
• D
  $\left( {G\frac{{{m_1}{m_2}}}{{{r^2}}} \cdot \frac{1}{{{m_1} + {m_2}}}} \right)\,t$