Setting up Google reCAPTCHA in a ReactJS app – eMumbahttp://emumba.com/blog/2016-12-07-setting-up-google-recaptcha-in-a-reactjs-app/
Using CAPTCHAs to Index Cultural Artifacts | Request PDFhttps://www.researchgate.net/publication/221460791_Using_CAPTCHAs_to_Index_Cultural_Artifacts
Why CAPTCHAs have gotten so difficulthttps://www.theverge.com/2019/2/1/18205610/google-captcha-ai-robot-human-difficult-artificial-intelligence
Why Captchas have gotten so difficult | Hacker Newshttps://news.ycombinator.com/item?id=19069365