An insulating thin rod of length l has a linear charge density ρ(x)=ρ0lx on it. The rod is rotated about an axis passing through the origin ( x=0 ) and perpendicular to the rod. If the rod makes n rotations per second, then the time averaged magnetic moment of the rod is:
a. πnρl3
b. 3πnρl3
c. 4πnρl3
d. nρl3