The electric field in an air filed rectangular waveguide at 3 GHz is given by the expression

E₂₅ (x, y, z) = 15 sin (5 πx) sin (12.5 πy) e^-hβz V/m. If the cross-sectional dimensions of the waveguide are a = 20 cm and b = 8 cm, then the mode of propagation is