According to Bohr's theory, the time averaged magnetic field at the center (i.e. nucleus) of a hydrogen atom due to the motion of electrons in the nth orbit is proportional to : (n = principal quantum number)

• A.  \[{{n}^{-4}}\] 
• B.  \[{{n}^{-5}}\]  
• C.  \[{{n}^{-3}}\] 
• D.  \[{{n}^{-2}}\]