The impedance of a circuit,when a resistance $R$ and an inductor of inductance $L$ are connected in series in an $AC$ circuit of frequency $f$ is

• A
  $\sqrt{R + 2\pi^2 f^2 L^2}$
• B
  $\sqrt{R + 4\pi^2 f^2 L^2}$
• C
  $\sqrt{R^2 + 4\pi^2 f^2 L^2}$
• D
  $\sqrt{R^2 + 2\pi^2 f^2 L^2}$