google 人机身份验证
Are you a robot? Introducing “No CAPTCHA reCAPTCHA”
https://googleonlinesecurity.blogspot.com/2014/12/are-you-robot-introducing-no-captcha.html
reCAPTCHA google 人机身份验证
Protect your website from spam and abuse while letting real people pass through with ease.
Tough on bots ，Easy on humans。
示例图片：
1 demo
https://www.google.com/recaptcha/api2/demo
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参考链接：
https://www.google.com/recaptcha/admin#list
https://www.google.com/recaptcha/intro/index.html
https://www.google.com/intl/zh-CN/policies/terms/
https://www.google.com/intl/en-US/policies/terms/
https://en.wikipedia.org/wiki/ReCAPTCHA
https://zh.wikipedia.org/wiki/ReCAPTCHA
其他参考链接：
https://github.com/google/recaptcha
https://wordpress.org/plugins/wp-recaptcha/
https://wordpress.org/support/register.php
https://www.drupal.org/project/recaptcha
http://contactform7.com/recaptcha/
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