Fill in the blanks with the correct letters.

Here are the answers filled in:

* You may be surprised to discover that not everyone wants to hear your music. Fortunately, there is a very useful invention that can help - headphones. C. They change an electrical signal into a sound in the user's ear. Of course, you will be familiar with headphones, but do you know how long they have been around?
* The first commercially successful set was designed by Nathaniel Baldwin in 1910 in the United States. E. But letters written by Baldwin show that they probably weren't the first.
* It is possible that the first headphones were created in the 1880s and worn by telephone operators. As the telephone moved into more people's homes, the telephone companies started to offer a new service. F. They phoned a special number, then the operator connected them to the theatre. They put on their headset, which was worn under the head, not over, and listened to the opera or a play from their homes. A.