Not allow learner to move on until exploring all choices

I am very new to Storyline.  I have created a 'main' page with 4 different choices. I have set up the triggers so that when the learner selects each choice, they will go to the slides associated with it, then back to the 'main' page.  I would like them to have to explore all four choices before they can move on.  How can I set this up?

Thanks!

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