[originaltext] Some psychologists maintain that mental acts such as thinking

Some psychologists maintain that mental acts such as thinking are not performed in the brain alone, but that one’s muscles also participate. It may be said that we think with our muscles in somewhat the same way as we listen to music with our bodies.
    You surely are not surprised to be told that you usually listen to music not only with your ears but also with your whole body. Few people can listen to music that is more or less familiar without moving their body or more specifically, some parts of their body. Often when one listens to a concert on the radio, he is tempted to direct the orchestra even though he knows there is a competent conductor on the job.
    Strange as this behaviour may be, there is a very good reason for it. One cannot derive all possible enjoyment from music unless he participates, so to speak, in its performance. The listener "feels" himself to the music with more or less noticeable motions of his body. The muscles of the body actually participate in the mental process of thinking in the same way, but this participation is less obvious because it is less noticeable.
Questions 19 to 21 are based on the passage you have just heard.
19. According to the passage, what do some psychologists maintain about thinking?
20. Why do people move their body while listening to familiar music?
21. What is the passage mainly about?