My solution was just a proof of concept to point you in the right direction.
you may remove the relevance settings for A,B,C and D.
Also the reset button should not be that hard, or maybe add a second column with "N" No as the value.
That way you would be able to choose.
Also you could use checkboxes and limit the maximum answers to two.
That way one could only check two answers.
If all this is not what you where looking for, I suppose holch's answer is the best so far.
Just create the subquestions a s separate questions, only visible if you chose something from the main question.
you may remove the relevance settings for A,B,C and D.
Also the reset button should not be that hard, or maybe add a second column with "N" No as the value.
That way you would be able to choose.
Also you could use checkboxes and limit the maximum answers to two.
That way one could only check two answers.
If all this is not what you where looking for, I suppose holch's answer is the best so far.
Just create the subquestions a s separate questions, only visible if you chose something from the main question.