A rod of length l and radius r is joined to a rod of length l/2 and radius r/2 of same material. The free end of small rod is fixed to a rigid base and the free end of larger rod is given a twist of θ, the twist angle at the joint will be
(a) θ/ 4
(b) θ/2
(c) 5θ/ 6
(d) 8θ/ 9