The making of glass is a very old industry---at least 4,500 years old. Glass has many extraordinary qualities and it is often being used in new ways. One of the most interesting new uses for glass is in telephone communication. Scientists have developed glass fibers as thin as human hair which are designed to carry light signal. When the light reaches the other end,it is first changed into electrical signals , which are in turn changed into sound messages. Called lightwave communication, the new system was used successfully in an experiment in Chicago in 1977. During the experiment, two glass fibers were able to carry 672 conversations at the same time. The lightwave cable , containing 144 glass fibers, is able to carry 50,000 conversations at the same time. The lightwave communication system has two important advantages. First, the glass fiber cables are smaller and weigh less than copper cables. Second, they cost less. Perhaps it can be said that telephone communication has entered the age of light. According to the passage the new telephone communication system   _  .
has been put into actual use was put into actual use in 1977 was put on experiment in 1977 had been repeatedly experimented on before 1977