Two coils are made of copper wires of the same length. In the first coil the number of turns is 3n and radius is r. In the second coil number of turns is n and radius is 3r the ratio of self-inductances of the coils will be:-
(A) 9 : 1
(B) 3 : 1
(C) 1 : 3
(D) 1 : 9
