Two rectangular waveguide have dimensions of $$1 cm \times 0.5 cm$$ and $$1 cm \times 0.25 cm$$ respectively. Their respective cut-off frequencies will be

A

15 GHz and 30 GHz

B

30 GHz and 60 GHz

C

15 GHz and 15 GHz

D

30 GHz and 30 GHz