tooth 牙齿（单数）
teeth 复数
1、The residents are fighting tooth and nail to stop the new development.
居民为制止新的`建房开发计划正在全力以赴进行斗争。
2、When did she cut her first tooth?
她什么时候长出的第一颗牙？
3、A high proportion of five-year-olds have teeth in poor condition.
五岁儿童牙齿不健康的比例高。
4、The dentist may decide that the wisdom teeth need to be extracted.
牙医可能会认为智齿需要拔掉。
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