A rod of radius ' r ' & length ' l ' is beaten such that its length doubles . If it took $\;t_{0}\;$ seconds to transfer heat Q , how much time will it take now ?

Question

$(a)\;t_{0}\qquad(b)\;2t_{0}\qquad(c)\;4t_{0}\qquad(d)\;\large\frac{t_{0}}{2}$