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Two particles of masses $M_{1}$ and $M_{2}\left(M_{1}>M_{2}\right)$ attract each other with a force inversely proportional to the square of the distance between them. The particles are initially at rest and then released. The centre of mass relative to a stationary observer

1. moves towards $M_{1}$
2. move towards $M_{2}$
3. remains at rest
4. moves with a speed proportional to $\sqrt{\frac{\mathrm{M}_{1}}{\mathrm{M}_{2}}}$