Dynamic Dropdowns: Change Sub-Department options based on selected Main Department

i want to create only 2 dropdown menus "main department" and "sub department" ..

i want if i want to select ehsaas centre then ehsas center and writing center should appear in sub department.

i dont want to have sub department dropdown for every main department.

its possible ? 

That's very much possible with Dynamic Dropdowns widget.

Click on Add Form Element > Widgets tab > then search for Dynamic Dropdowns

In the widget's settings, put all your Main Departments. If you want to place it's Sub Department, put the sub department right below the main department and indent it with space.

Follow the guide in the widget's settings and you'll be good to go.

Here's a sample of how I did it.

Hope this helps.