By default you'll have a Snippet called 'comment_spam_block' which will provide a way to ask your site commenters a simple question.
<r:random>
<r:error on="spam_answer"><p style="color:red">Answer <r:message /></p></r:error>
<r:option>
<p><label for="comment_spam_answer">What day of the week has the letter "h" in it's name?</label> (required)<br />
<r:spam_answer_tag answer="Thursday" /></p>
</r:option>
<r:option>
<p><label for="comment_spam_answer">Yellow and blue together make what color?</label> (required)<br />
<r:spam_answer_tag answer="green" /></p>
</r:option>
<r:option>
<p><label for="comment_spam_answer">What is SPAM spelled backwards?</label> (required)<br />
<r:spam_answer_tag answer="MAPS" /></p>
</r:option>
</r:random>
The snippet takes advantage of the built-in Radius tags r:random and r:option to provide a random
and less predictable selection of questions for commenters.
Be sure to alter this snippet for your site!
In your site layout (or another appropriate place) use the following code to display your comment form:
<r:snippet name="comments" />
Relative urls will not work on comment pages if they fail validation, since the page gets re-rendered at a (probably) different level of the hierarchy. Always use absolute urls and you won't have any issues.