Read the passage carefully and choose the correct answer.

At the beginning of TV history, there were several types of TV technology. One system was a mechanical model based on a rotating disc. (Rotating discs are discs that spin like CDs.) The other system was an electronic model. In 1906, Boris Rosing built the first working mechanical TV in Russia. In the 1920s, John Logie Baird in England and Charles Francis Jenkins in the United States demonstrated improved mechanical systems. Philo Taylor Farnsworth also showed an electronic system in San Francisco in 1927. His TV was the forerunner of today's TV, which is an electronic system based on his ideas.

Now TV is everywhere. Before 1947, there were only a few thousand televisions in the U.S. By the 1990s, there were televisions in 98% of American homes.

Rotating means _____