Two rods of equal cross - section have their ratio of lengths 1 : 2 . If the ratio of their coefficient of thermal conductivity are in the ratio 3 : 4 , the ratio of time taken for heat transfer for same temperature differance is -

Question

$(a)\;3 :8\qquad(b)\;8 :3 \qquad(c)\;2 : 3\qquad(d)\;3 : 2$