Captcha
A CAPTCHA (IPA:
/?kæpt??/) is a type of challenge-response
test used in computing to determine that the response is not generated by a computer.
 The
process involves one computer (a server) asking a user to complete a simple test
which the computer is able to generate and grade. Because other computers are
unable to solve the CAPTCHA, any user entering a correct solution is presumed
to be human. A common type of CAPTCHA requires that the user type the letters
of a distorted image, sometimes with the addition of an obscured sequence of
letters or digits that appears on the screen.
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