The biggest flaw with all CAPTCHA systems is that they are, by definition, susceptible to attack by humans who are paid to solve them.
所有的CAPTCHA验证系统都有一个最大的瑕疵：顾名思义，它们在人类本身的进攻面前不堪一击，有人付款给这些人破解它们。
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.
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。
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