Two particles of equal mass \( m \) move in a circle of radius \( r \) under the action of their mutual gravitational attraction. The speed of each particle will be:
\( \sqrt{\frac{4Gm}{r}} \)
\( \sqrt{\frac{Gm}{4r}} \)
\( \sqrt{\frac{Gm}{r}} \)
\( \sqrt{\frac{Gm}{2r}} \)