This is a comparison of online services which helps to prevent
automated abuse of your site (such as automatic comment spam or
registrations) by using a CAPTCHA to ensure that only humans
perform certain actions.
While these services helps to reduce spam, they will not totally
prevent it, since some humans add manually their comments.
These services usually shows a text in an image and visitors
must type it to validate the form. To ensure accessibility, some
services provide an audio CAPTCHA so that the visitors can
listen to it, if they are unable to see the image.
A dedicated comparison has been created for services with
advertisements solutions combined:
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