HD:

$\begin{align}

  & R=\frac{\rho \times l}{S} \\

 & \Rightarrow \rho =\frac{R\times S}{l} \\

 & \frac{{{R}_{1}}\times {{S}_{1}}}{{{l}_{1}}}=\frac{{{R}_{2}}\times {{S}_{2}}}{{{l}_{2}}} \\

 & \Rightarrow {{l}_{2}}=\frac{{{R}_{2}}\times {{S}_{2}}\times {{l}_{1}}}{{{S}_{1}}\times {{R}_{1}}}=\frac{8{{R}_{1}}\times 2{{S}_{1}}\times {{l}_{1}}}{{{S}_{1}}\times {{R}_{1}}}=16{{l}_{1}} \\

\end{align}$

Chọn đáp án A