28. waveguides are used mostly for microwave signals because (a) they depend on rectilinear propagation which applies to microwaves only (b) losses are too heav

28. Waveguides are used mostly for microwave signals because (A) They depend on rectilinear propagation which applies to microwaves only (B) Losses are too heavy at other frequencies (C) They are not excited at lower frequencies (D) They would be too bulky at lower frequencies