A、In the later 1920s. B、In the mid-1950s. C、In the 1970s. D、In the mid-1980s. B

Ever since Philo T. Farnsworth assembled the first television set in his Indiana garage in 1927, the basic technological principles for bringing electronic pictures into the home have remained the same. There have been only two major changes in the way TV sets work: the introduction of color in 1954, and the shift from tubes to transistors in the 1970s.
    Now a radical change is about to take place. Digital television—which uses a different method of signal transmission   will significantly alter the way future television sets will look and perform.
    The digital set, already on sale in Europe and scheduled to be introduced in the United States this fall, is a cross between a computer terminal and a TV set. Although the differences it will bring may not be dramatic, its improved quality will be increasingly appreciated, as zoom effects, stereo sound, and freeze-frame views of live shows become commonplace. Digital TV promises to give viewers a clearer, more consistent picture than has been available so far.