question: What is the definition of reciprocal processes in time-invariant systems? option 1: A system is reciprocal if the S-parameters are symmetric option 2: A system is reciprocal if the two reaction integrals are equal option 3: A system is reciprocal if the permittivity is globally modulated option 4: A system is reciprocal if the material properties vary harmonically with time option 5: A system is reciprocal if the phase constants are the same for two modes 