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Select the Class field in the body of the panel.
Drag a dropdown from the toolbox and drop it on the field: 
The field turns into a Dropdown:
Next you will add a set of fixed values to the drop down.
Click the Edit button next to FixedValues:
Click on the Plus button to add a value to the drop down:
Using the Plus button add these values:
When you are done, click OK.
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