infrequent: implies occurrence at wide intervals in space or time: family visits that were ~ and brief. 又如an infrequent guest/ buses稀客/班次少的公交车
uncommon: suggests something that occurs or is found so infrequently as to be exceptional or extraordinary: smallbox is now ~ in many countries. exceptional罕见的特殊的例外的，这里和extraordinary完全同义 天花
scarce: implies a falling short of a standard or required abundance: jobs were ~ during the Depression.
rare: suggests extreme scarcity or infrequency and often implies consequent high value: ~ first editions of classics fetch high prices. (rare earth/metal/bird稀土/稀有金属/珍禽，如the air is rare at high altitudes高处空气稀薄，如it's rare to meet someone who's content with their life对生活满足的人很少见)
sporadic: implies occurrence in scattered instances or isolated outbreaks: ~ cases of the genetic disorder. (break out突发突然爆发to start suddenly, especially of war, fighting, or similarly undesirable things战争战斗等不良事件突然发生，如a fire/riot/war had broken out突发了火灾/暴乱/战争)
infrequent不常: 指某事物在时空上间隔远，uncommon不寻常杰出: 表示少见到显得不寻常，scarce缺乏: 指少于需要的标准，rare罕见珍稀: 表示极为少见缺乏，因而有价值，sporadic偶发: 指分散孤立地发生出现
记忆方法: 1）首字母IUSRS重新排序成US IRS是US Internal Revenue Service美国国家税务局<==罕见 因为在外国
2）罕见的意思是不常见不充足mean not common or abundant.
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