题目内容：
They are designed to help _1__ their skills,ability and appreciation of life; to give them a chance to practice leadership and __2_ responsibilities;_3__ school courses;and to provide additional outlets and stimuli.There is often a _4__ of activities from which to choose,such as nature clubs,musical organizations,science clubs,art and drama groups,or language clubs.A． _5__ selection of sport activities is always available.__6_ every school has a student-run newspaper;often a photographic darkroom is also _7__,some of these activities take place during the school day,but _8__ are held after classes are over._9__ they’re optional they _10__ a part of the A．merican educational experience. Parents encourage their children to participate in those program that __11_ suit their own special talents and interests.Much is learned during these off-duty hours,especially in _12__ of human "give-and-take".A．mericans believe this _13__ human relationships,social skills and a well-trained body,_14__ intellectual development .B．oth _15__ and college admission officers in the United States carefully consider the extra curricular activities _16__ students have participated,both during their free time after school and also during the long holidays.These indicate to them _17__ of a young person’s leadship potential enthusiasm,creativity,_18__ of interest,vitality and personality.They weigh these qualities,together with the academic record,in order to _19__ a student’s intelligence,perseverance and ability to use what he knows,_20__ merely repeating it by rot on examination papers.根据以上短文，回答题目
broaden
extend
stretch
concentrate
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