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Từ điển Oxford Advanced Learner 8th 
	
		  hard nut   
  ▪ I. a hard/tough ˈnut idiom   (informal)a person who is difficult to deal with or to influence     Main entry: ↑nutidiom    ▪ II. a hard/tough ˈnut (to ˈcrack) idiom  a difficult problem or situation to deal with     Main entry: ↑nutidiom
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