Correct Answer - D
Let the heat transfered be Q.
when rods are joined end to end. Heat transfered by each rod
`Q = (K(2A)Deltatheta)/l xx 12` .....(i)
when rods are joined lengthwise,
`Q = (KADeltatheta)/(2l) t` .......(ii)
As the same amount of heat is transfered
`therefore` From Eqs. (i) and (ii) , we get t = 48 s