11. A short solenoid (length I, radius a, ni turns per unit length) lies on the axis of a very one solenoid (length /, radius b, na turns perunit length), where a＜b. What is the total magnetic flux through the long solenoid if a steady current / flows in the shortaDsolenoid?
(A) μ0πa2n1n2ll
(B) μ0πb2n1n2ll
(C) μ0πa2n12ll
(D)μ0πabn1n2ll