I've tried whole day for this.
I'd like to make 2 combo boxes.
1st is category.
2nd is subcategory.

1st step, select item in 1st combo the category then
add items in the 2nd combo the subcategory for each index of category's items.

Ex. If index of category's item is 1 then add ("hi","hello","bye") to subcategory's items

If index of category's item is 2 then add ("1","2","3") to subcategory's items

What could the code be?