单选题 Many deaf people in the United States do use a special telephone system. It is called TDD. The letters TDD represent the words Telecommunication Device for the Deaf.
One kind of TDD looks like a small typewriter. It has rows of buttons. Each button is marked with a letter. Messages are spelled out by pushing the correct buttons. The TDD connects to a telephone through a device called a modem. The modem changes electronic signals from TDD into sounds the telephone can understand. Some TDDs produce messages on paper. Others show them on a television or computer screen.
For example, a deaf gift named Linda wants to call her boy friend, Mark, who is also deaf. First, Linda puts the telephone receiver into the modem. Then she calls Mark"s telephone number by pushing the correct numbered buttons on the telephone. A light on her TDD shows that Mark"s telephone is ringing.
A different light will show if Mark"s telephone is used. Linda will have to wait to call later. If Mark"s telephone is not used, he will see a light going on and off. Then he will know that some one is trying to call him.

单选题 According to the passage, TDD is ______.

单选题 How does TDD work according to the passage?

单选题 How does the deaf person know that there is a call for him?

单选题 How do the deaf people use the TDD?