A proton and an alpha particle move in circular orbits in a uniform magnetic field. Their speeds are in the ratio of 9 : 4. The ratio of radii of their circular orbits $$\left( {{{{r_p}} \over {{r_{alpha}}}}} \right)$$ is

Assertion (A) : When radius of a current carrying loop is doubled, its magnetic moment becomes four times.

Reason (R) : The magnetic moment of a current carrying loop is directly proportional to the area of the loop.