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Good Question is a free simple WordPress plugin to stop spam comments and registrations on the site. 
Allows to add a question on the comments and registration form, easy for humans, but not solvable for the spam bots. User must select correct answers, otherwise registration will fail.
The best thing of this super simple plugin is that it 100% works!
The smartest bots that easily pass the Google reCAPTCHA are helpless against the Good Question. Just because they don’t know what to do with that. Bots are not trained pass the Good Question validation because it’s not so popular as Google reCAPTCHA or other services.
The Good Question plugin successfully protects WordPress sites from spam registrations since 2013 and up for today.
Key features of the plugin:
<ul>
<li>100% effective. There are no services to help the bot to answer the abstract question, so they can’t pass validation.</li>
<li>Compatible with the most of the WordPress themes, including all AppThemes themes.</li>
<li>Works on the Registration form.</li>
<li>Works with WordPress MU Sign Up form.</li>
<li>Works on the Comments form.</li>
<li>Simple and lightweight.</li>
<li>Allow to create your own unique Question and set of Answers.</li>
<li>Allow to edit all CSS properties for Question and each of Answers separately.</li>
<li>There is absolutely no difference of which question and how many answers you configure for a good question, just make it easy for your real visitors.</li>
<li>You can set colors or images through CSS properties, instead of simple text answers.</li>
</ul>

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