How can I set a default state for a drop down options list?

How can I set a default state for the checkbox in a drop down options list ?

The checkbox by the main title is checked in the preview, showing the dropdown list. I want to set it to unchecked. 

Thank you. 

When I opened your form, there is not product selected by default. It is possible that you already fixed this problem? Here's a screenshot:

I cloned your form and I found out that there is no item/product selected by default. Here's a screen capture:

If you're still having issues with the products, let us know. Thank you.