Variables to disable buttons

Hi everyone, I have a slide with five buttons that the learner is instructed to choose two of the five buttons.  Each button the learning clicks takes them to a layer with some information on it.  What I'm trying to do is disable the buttons that the learn does not click, while still allowing the use to go back and view the buttons they have chosen.  

I am able to get the buttons to disable after two clicks with a variable that counts the number of user clicks.  But for the life of me I can't figure out how not to disable the buttons the user clicked. I tried using a true/false variable to monitor if a button has been click and to not disable it but can't get it to work.  Anyone have a suggestion?

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