A、Because the old one could not transfer signals clearly. B、Because the old one could only send two messages along the same line

Thomas Edison is a well known American inventor. He was taught by his mother when he was a child. Later he learned things by himself by reading and experimenting. He built a lab for himself to work in, and he worked very hard in his sixty years of scientific activities. He made over a thousand inventions. Few inventors in the world have had such great success.
    Edison’s first invention was made in 1868. He was then a telegraph operator. He wished to work out a new method to improve the old telegraph system because it was impossible at that time for the system to send more than one message along the same line at the same time. He read all the works of Faraday, another great scientist, before starting his invention. Sometimes he didn’t go to bed and didn’t even stop to eat. He thought he had very much to do and there was no time to lose. Days and nights he was reading and reading. After he read through Faraday’s writings, he began his experiments. A few months later he succeeded in building a double transmitter and improved the old telegraph System.