What does they in paragraph 3 refer to?

Choose the letter A, B, C or D to answer these following questions
Telephones help people speak to one another when they are apart. For more than a hundred years, nearly every telephone was a landline. A landline telephone is one that needs to be connected by a wire to a network of other telephones. Because of the wires, people could not take those telephones with them when they left their homes or offices.
What if you had to make a call while you were away from home? You had to find a pay phone. Pay phones are landlines found in public places. Many pay phones are on the street. You can make a call from inside a glass or metal space called a phone booth. Once you are inside the booth, you put coins into a slot in the phone to make a call.
Telephones have seen a lot of progress. Today, many people carry cell phones. Cell phones do not need to be connected with wires. They can be used almost anywhere and can fit in an adult’s hand.
Many cell phones sold today are smart phones. A smart phone is a cell phone that has lots of computer-style features. For example, people use smart phones to check e-mail and go on the Internet. And all that can be done using something small enough to carry in a pocket!