Drag and Drop activity questions

I have inherited a Storyline 3 project from another developer and have been asked to makes some changes to it.  It is an activity introducing the different segments in our business.  

The user drags a house over to the corresponding scenario box that best describes the business segment (sorry, had to blur things out for confidentiality reasons.)  Right now it is very linear with the user starting with the top left house and proceeding through all 6 houses, with the user given correct and incorrect feedback.  Each house is grayed out and made unavailable to move once the user selects the correct scenario until they have used all of the houses.  It is using a free form drag and drop question to accomplish this.

I would like to change it up so that it doesn't matter which house the user starts with but still have the houses grayed out until they have used all of them.  I'm thinking I would need to assign a variable to each house and then use that variable to identify when a house has been used but I am getting stuck. 

Anyone with a good idea on how to accomplish this?

Scott