An alternating current of frequency f is flowing in a circuit containing a resistance R and a choke Z in series. The impedance of this circuit is

• A

  $\mathrm{R}+2\mathrm{\pi fL}$

• B

  $\sqrt{{\mathrm{R}}^2+4{\mathrm{\pi }}^2{\mathrm{f}}^2{\mathrm{L}}^2}$

• C

  $\sqrt{{\mathrm{R}}^2+{\mathrm{L}}^2}$

• D

  $\sqrt{{\mathrm{R}}^2+2\mathrm{\pi fL}}$