完成下列各题 In the United,States the most popular form. of folk dancing since the early days has been square dancing．In early times，when men and women worked in groups to 21________ a barn(谷仓)for harvest crops，they danced when the work was done．The music 22________ from a violin for the most part．But if there was no one to play all instrument(乐器)，clapping(击掌)was used to 23________ the rhythm(节奏)by which to dance．The early settlers(移民>danced in a store，in a barn，or in a farm kitchen． After some years 24 ________，square dancing became 25 ________ popular in cities and towns，but it 26________ popular in the country．Then it became popular in the cities once again·Today in 27 ________ Darts of the United States you will find some school，club，or other group that is square dancing for fl in． A square is formed by four．couples(对，双)who stand 28 ________the center of the square. Each couple stands on one 29________ 0f the square，the boy on the left and the girl on the right． The“caller”is an important part of the 30________ since he tells the dancers which steps to do． If the dancers do not know the steps，he teaches them．It 31________ time to learn to be a good caller，and good callers are always in demand． Large or small groups of people can dance at one time．Sometimes 800 or 1，000 people may be dancing at the 32 ________time．Or there may be only one square of 33________ people. Costumes(服装)are worn by some who square dance．This 34________ the dancing more colorful to watch．The costumes vary(不同)from place to place．Women often wear full skirts of various colors with pretty blouse(罩衫)．Men may have colored skirts and western trousers which they wear35________ when square dancing．