question: What is the key condition for achieving nonreciprocal propagation in a waveguide with travelling-wave modulation? option 1: The modulation frequency is smaller than the angular frequency of the second mode option 2: The modulation frequency is equal to the sum of the angular frequencies of the two modes option 3: The modulation frequency is larger than the angular frequency of the first mode option 4: The modulation frequency is equal to the difference between the angular frequencies of the two modes option 5: The modulation frequency is equal to the angular frequency of the second mode 