What is the effect of applied magnetic field on absorption spectrum?


> 
> Upon application of a weak magnetic field, a line in the microwave absorption spectrum of rigid rotor splits into $3$ lines. The quantum number $(J)$ of the rotational energy level from which the transition originates is
> 
> 
> (**a**) $0$  
> 
> (**b**) $1$  
> 
> (**c**) $2$  
> 
> (**d**) $3$
> 
> 
> 


Can anyone explain the answer (**a**) for this?
For rotational levels, each level is $(2J+1)$ fold degenerate. According to that, the answer should be (**b**).


