extend: implies a drawing out of extent in space or time and may also imply increase in width, scope, area, influence, or range: ~ welfare services. 这里用extent解释extend，属于自释 extent是所有范围的基础词，对range的解释是range is the general term indicating the extent，即range泛指extent
lengthen: suggests an increase of length in either time or space: ~ the school year. 自释 延长针对长度，扩展针对范围
elongate: usually implies increase in spatial length and frequently suggests stretching: the dancer's ability to ~ her body.
prolong: suggests chiefly increase in duration especially beyond usual, normal, or pleasing limits: ~ed illness. (duration持续时间期间the length of time that something lasts or continues，如a contract of three years' duration三年期的合同，如the school was used as a hospital for the duration of the war战争期间这所学校被用作医院)
protract: adds to prolong implications of needlessness, vexation, or indefiniteness: ~ed litigation. （vex让人烦恼困惑to annoy, worry, or perplex someone，如a vexing problem令人烦恼的问题，如a problem that vexes cancer researchers让癌症研究人员烦恼的问题）
extend扩展: 指扩展时空中的范围，或增加宽度领域面积影响范围等，lengthen延长: 表示增加时空中的长度，elongate伸展: 通常指增加空间长度，常暗示拉伸，prolong延长拖延: 多指延长时间长度，超出正常合理适宜的限度，protract延长拖延: 比prolong多了不必要不确定让人烦恼的意思
记忆方法: 1）首字母ELEPP大象<==延长
2）延长的意思是扩展或增加长度mean to draw out or add to so as to increase in length.
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