Impedance of a given series LCR circuit, fed with alternating current, is the same for two frequencies $$f_1$$ and $$f_2$$. The resonance frequency $$f_R$$ of the circuit is

A

$$\frac{f_1+f_2}{2}$$

B

$$\frac{2f_1f_2}{f_1+f_2}$$

C

$$\sqrt{f_1f_2}$$

D

$$\sqrt{f_1^2+f_2^2}$$