Hi, Marie-France,
this is a nice paper-pencil questionnaire, but - as often -, you can draw a lot, but it is difficult to realize as an web based survey.
Well, in my opinion there are two solutions available at the moment here in the forum.
The first is based on the idea that it is not necessary to check "opportunity" and "participation" independently.
If somebody participated he had the opportunity. So we can check these two options as radio buttons.
The second let's you select both (just to show it)
Then you have to validate that "hours" is only entered if participated,
and in the second case that "participation" is only entered if "opportunity".
If you think this could suit your wishes, I'll send an LSS file of this (has to be improved a bit).
Joffm
One addition:
Improvement of first example: