excel - list appended to list

The drop-down 1 selection should output a predefined list somewhere and drop-down 2 selection should append this list with the contents of another predefined list. For example:

from drop down 1 I can select a, b or c. Selecting a gives me the list 1, 2, 3 selcting b gives me i, ii, iii and selecting c gives me I, II, III outputted somewhere in the same worksheet.

then from drop-down 2 I can select x, y or z. Selecting x appends 99, 100, 101 to the end of the list I got from selecting something in drop down 1

an example outout:
1
2
3
99
100
101

but I need to be able to change the outputted lists. So for example selecting a from drop-down 1 gives me 1, 2, 3, 4 or just 1, 2

I'm grateful for any help.

Drop down on a form or from a range of cells?

from a range of cells