Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions from 16 to 22. 

     The term "folk song" has been current for over a hundred years, but there is still a good deal of disagreement as to what it actually means. The definition provided by the International Folk Music Council states that folk music is the music of ordinary people, which is passed on from person to person by being listened to rather than learned from the printed page. Other factors that help shape a folk song include:  continuity (many performances over a number of years): variation (changes in words and melodies either through artistic interpretation or failure of memory); and selection (the acceptance of a song by the community in which it evolves). 

     When songs have been subjected to these processes, their origin is usually impossible to trace. For instance, if a farm laborer were to make up a song and sing it to a couple of friends who like it and memorize it, possibly when the friends come to sing it themselves, one of them might forget some of the words and make up new ones to fill the gap, while the other, perhaps more artistic, might add a few decorative touches to the tune and improve a couple of lines of text. If this happened a few times, there would be many different versions, the song's original composer would be forgotten, and the song would become common property. This constant reshaping and re-creation is the essence of folk music.  Consequently, modem popular songs and other published music, even though widely sung by people who are not professional musicians, are not considered folk music. The music and words have been set by a printed or recorded source, limiting scope for further artistic creation. These songs' origins cannot be disguised and therefore they belong primarily to the composer and not to a community. 

     The ideal situation for the creation of folk music is an isolated rural community. In such a setting folk songs and dances have a special purpose at every stage in a person's life, from childhood to death. Epic tales of heroic deeds, seasonal songs relating to calendar events, and occupational songs are also likely to be sung.

The word "essence" in paragraph 2 is closest in meaning to ______.