Câu hỏi:

Read the passage and choose the best option to each question

Since the dawn of time, people have found ways to communicate with one another. Smoke signals and tribal drums were some of the earliest forms of communication. Letters, carried by birds or by humans on foot or on horseback, made it possible for people to communicate larger amounts of information between two places. The telegraph and telephone set the stage for more modern means of communication. With the invention of the cellular phone, communication itself has become mobile.           For you, a cell phone is probably just a device that you or your friends use to keep in touch with family and friends, take pictures, play games, or send text messages. The definition of a cell phone is more specific: it is a hand-held wireless communication device that sends and receives signals by way of small special areas called cells.           Walkie-talkies, telephones, and cell phones are duplex communication devices: they make it possible for two people to talk to each other. Cell phones and walkie-talkies are different from regular phones, because they can be used in many different locations. A walkie-talkie is sometimes called a half-duplex communication device, because only one person can talk at a time. A cell phone is a full-duplex device because it uses both frequencies at the same time. A walkie-talkie has only one channel. A cell phone has more than a thousand channels. A walkie-talkie can transmit and receive signals across a distance of about a mile. A cell phone can transmit and receive signals over hundreds of miles. In 1973, an electronic company called Motorola hired Martin Cooper to work on wireless communication. Motorola and Bell Laboratories (now AT&T) were in a race to invent the first portable communication device. Martin Cooper won the race and became the inventor of the cell phone. On April 3, 1973, Cooper made the first cell phone call to his opponent at AT&T while walking down the streets of New York City. People on the sidewalks gazed at Cooper in amazement as he walked down the street talking on his cellular phone. Cooper’s phone was called Motorola Dyna-Tac. It weighed a whooping 2 ½ pounds (as compared to today’s cell phones that weigh as little as 3 or 4 ounces).           After the invention of his cell phone, Cooper began thinking of ways to make the cell phone available to the general public. After ten years, Motorola introduced the first cell phones for commercial use. The early cell phone and its service were both very expensive. The cell phone itself cost about $3,500. In 1977, AT&T constructed a cell phone system and tried it out in Chicago with over 2,000 customers. In 1981, a second cellular phone system was started in the Washington, D.C. and Baltimore area. It took nearly 37 years for cell phones to become available for general public use. Today there are more than sixty million cell phone customers with cell phones producing over thirty billion dollars per year.

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