For a resistance $$R$$ and capacitance $$C$$ in series, the impedance is twice that of a parallel combination of the same elements when used with an AC voltage of frequency $$f$$. The frequency $$f$$ of the applied emf is

A

$$f = 2\pi RC$$

B

$$f = \frac{1}{2\pi RC}$$

C

$$f = \frac{2\pi}{RC}$$

D

$$f = \frac{1}{2\pi\sqrt{R^2 + C^2}}$$