creating an 100 question exam using Storyline 2 - issues with changing out the submit button and using the next button.

Hello, I have spent 3 days trying to get things to work.  The customer wants a 100 question exam. This exam once it is built will be the template for 5 more of same. They are for a certification to be earned.  No problem can do. However, they want to be able to go back and change a question after answering it.  So, I changed out the submit button to the next button, changed the slide properties to "When visiting" "Resume saved state". Allowing the user to go to the navigation-menu set as "Restricted" so they can go back to a previously visited question - and give them the opportunity to change their answer.  then the scoring is at the end on the "Results" slide. Works like a charm!... The issue? You can select the "Next" button without making a selection on the MCQ or TF Or Multi-Answer. etc.  I have tried changing the triggers on the Next button many different ways.  I tried changing the state, I've tried changing the values, I am not strong on the triggers and  I cannot get it to work. Now it's 2 to 3 days later...have lost precious time and I'm still dead in the water with this. Please give me some suggestions or trigger knowledge or best practices...... I'm almost to the point where I will have to tell the customer that this cannot be done... the software will not allow it.... Their old original Exams  were create a long time ago in Lectora...which gave them this functionality with a previous button to allow them to go back and change a previous question. The users who take these quizzes are used to this type of availability....