Respuesta :
As we know that the conductance is given by the formula
[tex]k = \frac{A}{\rho L}[/tex]
now here we know that
[tex]A = \pi r^2[/tex]
L = length
now we know that conductance will remain constant while the length is quadrupled
so here we have
[tex]k_1 = k_2[/tex]
[tex]\frac{A_1}{\rho L_1} = \frac{A_2}{\rho L_2}[/tex]
[tex]\frac{\pi r_1^2}{\rho L_1} = \frac{\pi r_2^2}{\rho 4L_1}[/tex]
[tex]4 r_1^2 = r_2^2[/tex]
[tex]r_2 = 2r_1[/tex]
so here radius becomes double
so correct answer will be
[tex]2r[/tex]