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Ugly duckling 丑小鸭 (来源：英语学习门户网站EnglishCN.com)

An ugly woman dreads the mirror. 丑妇怕照镜子

under the hammer 被拍卖

under one's nose 在眼皮底下

Under water, famine; under snow, bread. 洪水造饥荒；瑞雪照丰年

under way 在进行中

under the weather 身体不适

Unequal marriages are seldom happy. 不平等的婚姻不会幸福。

United we stand, divided we fall. 团结则存，分裂则亡。

Unpleasant advice is a good medicine. 逆耳之言是良药。

Up corn, down horn. 谷贵则肉贱

ups and downs 沉浮；兴衰

up to scratch 达到标准

upset the apple cart 美梦破灭

Use a book as a bee does a flower. 用书如蜜蜂采蜜

The used key is always bright. 常用的钥匙不生锈。