An air filled rectangular waveguide with dimensions a = 75 mm, b = 37.5 mm has same guide wavelength at frequencies $$f_1$$ and $$f_2$$ when operated at $$TE_{10}$$ and $$TE_{20}$$ modes respectively. If the frequency $$f_1$$ is $$\sqrt{13}$$ GHz, what is frequency $$f_2$$ in GHz?

A

10

B

5

C

$$\frac{\sqrt{3}}{2}$$

D

$$2\sqrt{13}$$