What is CAPTCHA?
CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a type of challenge-response test used in computing to determine whether the user is human.
Instruction: What is CAPTCHA?
Context: 
Response: CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a type of challenge-response test used in computing to determine whether the user is human.