Since you're using Office 2013, I'd recommend eschewing formfields and using content controls. With these, some custom Quick Parts (building blocks) and a bit of code, you can have Word conditionally insert the desired dropdowns without having to code for populating them as well. Conditionally inserting & populating dropdown formfields takes a whole lot more work.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]