In what year did U. S. television broadcasting stations switch their over-the-air transmission of audiovisual signals from analog to digital technology

Two students are discussing electricity that has a frequency of 60 hz. student a says that this type of electricity is referred to as ac. student b says that in this type of electricity, the electrons flow in only one direction. which of the following statements is correct? a. only student a is correct b. only student b is correct c. both of the two students are correct d. neither of the two students is correct