According to Bohr theory, in the ground state of hydrogen atom, an electron revolves in circular orbit of radius $$r$$ with velocity $$v$$ and circulation frequency $$f$$. The magnetic dipole moment of the electronic orbit is $$p_m$$. The magnetic field produced by the circulating electron at the center of the atom is $$B$$. Then for a $$\mathrm{He}^+$$ ion in the state $$n=2$$,

A

the radius of the orbit is $$2r$$

B

the magnetic dipole moment of the orbit is $$2p_m$$

C

the frequency of circulation is $$\frac f2$$

D

the magnetic field at the center is $$\frac B4$$