Fade background when an object with hidden marker is clicked on

For this week's challenge #118 Creating Labeled Graphic Interactions for E-Learning, I used Jerson's idea of using an object as the marker and hiding the marker.

Issue:  I want to know how to fade out the other images on the slide when one of the objects is selected.  Keep in mind that the marker has been set up to display when the object is clicked on.  I have tried to use layers and hotspots but have not been successful yet. 

I hope someone can help me. 

Jerson's video:  https://youtu.be/JbU8oke_jmI

 

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