mm_forum - which captcha

For a project I installed mm_forum. It is nice, it works more or less out of the box. At one point I get stuck. At the registration process I need a captcha function. In the configuration of mm_forum is a checkbox with whom you can activate the use of a captcha extension but it was not written which extension this could be. There are about 10 extension with captcha functionality.

At least I do understand it. There was written, the "captcha" extension, what pointed out, that there is an extension with the extension key "captcha". And it was ...
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this one
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Title "Captcha Library", extension key "captcha", version "1.1.1", last update "03-12-09", from "Kasper Skårhøj" and downloaded "20.555" times.

So I installed it, don't changed anything at the installation process. It works out of the box with mm_forum.