Ok I've found the script but have no idea where to put it ?
I've tried to insert it in the custom javascript of my theme but didn't work.

Thank you for your help.

If you found this
[url] forums.limesurvey.org/forum/can-i-do-thi...n-as-exclusive-optio [/url]
there is a sample survey attached in tpartner's answer.
Import and study it.
You may neglect the first script which randomizes the columns.

And you also find a solution in the manual. "Workarounds uising javascript"
[url] www.limesurvey.org/manual/Workarounds:_M...ime_using_Javascript [/url]
Example 42.2. (similar solution)
Here you are also told where to put the code.

Joffm