$A$ tapered bar of length $L$ and end diameters $D_1$ and $D_2$ is made of a material of electrical resistivity $\rho$. The electrical resistance of the bar is

  • A
    $\frac{4 \rho L}{\pi(D_1+D_2)^2}$
  • B
    $\frac{4 \rho L}{\pi(D_1-D_2)^2}$
  • C
    $\frac{\rho \pi \sqrt{D_1 D_2}}{4 L^2}$
  • D
    $\frac{4 \rho L}{\pi D_1 D_2}$

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