An air filled rectangular waveguide $$R_{1}$$ is operating at the frequency 2 GHz and another air filled rectangular waveguide $$R_{2}$$ is operating at 4 GHz. The guide wavelengths of these waveguides at their respective frequencies are equal. If the cut-off frequency of waveguide $$R_{1}$$ is 1 GHz, what is the cut-off frequency of the waveguide $$R_{2}$$ in GHz?

A

$$\sqrt{10}$$

B

$$\sqrt{11}$$

C

$$\sqrt{12}$$

D

$$\sqrt{13}$$