Acylindrical waveguide with radius of 3.5 cm has waves travelling in $$TM_{12}$$ mode. The value of $$1^{st}$$ zero of $$2^{nd}$$ order Bessel function is 7. Find the cut-off wavelength for this mode.

A

$$\pi$$ cm

B

$$1.5 \pi$$ cm

C

$$2 \pi$$ cm

D

$$\frac{\pi}{2}$$ cm