当用户被要求输入验证码时，用户可以看到两个单词：一个是已知的单词（控制单词），另一个是OCR软件无法确认的单词。

When users solve a CAPTCHA through reCAPTCHA, the software will give users two words: one with a known answer (the control word) and one where the OCR software wasn't quite sure what the word was.

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每一个未知词作为验证码呈现给不同用户数次，确保它被正确辨认。

Each unknown word is presented as a CAPTCHA several times, to different users, to ensure that it has been read correctly.

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UI - patterns是另外一个顶级用户界面库，收集展示了一些改进过的设计模型，比如验证码、帐户注册、导航标签等。

UI-patterns is another top-notch User Interface library that collects and showcases reoccurring design patterns such as Captchas, Account Registrations, and Navigation Tabs.

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