JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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What you are looking for is a widget called Dynamic Dropdowns.
What is great about them is that it allows you to add layers of dropdowns, allowing you to quickly change the data in the dropdown under the other one as if you have set a lot of different conditions, but without actually having a need to do any.
If you would like to add it to your jotform, but are not sure how, please take a look here: How to Add a Widget to your Form
Do let us know how it goes.