Bài làm

$\text{R= }\!\!\rho\!\!\text{ }\frac{\text{l}}{\text{S}}\Leftrightarrow \text{R= }\!\!\rho\!\!\text{ }\frac{\text{l}}{\text{ }\!\!\pi\!\!\text{  }\!\!\times\!\!\text{ }{{\text{r}}^{\text{2}}}}\Leftrightarrow \text{R= }\!\!\rho\!\!\text{ }\frac{\text{l}}{\text{ }\!\!\pi\!\!\text{  }\!\!\times\!\!\text{ }{{\left( \frac{\text{d}}{\text{2}} \right)}^{\text{2}}}}\Leftrightarrow \text{R= }\!\!\rho\!\!\text{ }\frac{\text{4l}}{\text{ }\!\!\pi\!\!\text{  }\!\!\times\!\!\text{ }{{\text{d}}^{\text{2}}}}$

Đáp án A