Auto fill dependent drop-down list if only one choice

I have the attached my spreadsheet that requests new student machine orders.

I have many dependent drop-down lists in this spreadsheet which work as expected and they will clear if the parent drop-down options changes.

I am trying to do the following:

I would like to auto-populate a cell if there is only one option in the drop down list for that cell. I would like this function for all cells that have one option. Since each drop-down is dependent on another I am not sure how to make this work.

(Example: If I choose Apple iMac the OS column will automatically select Yosemite )

If there is an easier way to do this that the way that I have chosen I am open to suggestions.

Thanks in advance for any assistance.

Sorry Fotis. I fixed my post. Thanks for the note.

Thank you Bernie. This did it. I was researching this for days.