Read the following passage and mark the letter A. B. C or D your answer sheet to indicate the correct answer to each of the questions.

CAN ANIMALS TALK?

In 1977, a young Harvard graduate named Irene Pepperberg brought a one-year-old African gray parrot into her lab, and attempted something very unusual. At a time when her fellow scientists thought that animals could only communicate on a very basic level, Irene set out to discover what was on a creature's mind by talking to it. The bird, named Alex, proved to be a very good pupil.

Scientists at the time believed that animals were incapable of any thought. They believed that animals only reacted to things because they were programmed by nature to react that way, not because they had the ability to think or feel. Any pet owner would probably disagree. Pepperberg disagreed, too, and started her work with Alex to prove them wrong.

Pepperberg bought Alex in a pet store. She let the store's assistant pick him out because she didn't want other scientists saying later that she'd deliberately chosen an especially smart bird for her work. Most researchers thought that Pepperberg's attempt to communicate with Alex would end in failure.

However, Pepperberg's experiment did not fail. In fact, over the next few years, Alex learned to imitate almost one hundred and fifty English words, and was even able to reason and use those words on a basic level to communicate. For example, when Alex was shown an object and was asked about its shape, color, or material, he could label it correctly. He could understand that a key was a key no matter what its size or color, and could figure out how the key was different from others.

Pepperberg was careful not to exaggerate Alex's success and abilities. She did not claim that Alex could actually “use” language. Instead, she said that Alex had learned to use a two-way communication code. Alex seemed to understand the turn-taking pattern of communication.

The word "them" in paragraph 2 refers to ________.