1.Create a dashboard prompt and Put columnA and assgn it to a presentation variable say 'VAR_1'.
2.Now create one more dashboard prompt select SQL result in choice list .and enter logical sql like
FROM tab_name where
columnB < (
WHERE columnA= 'VAR_1')
3. put both in a dashboard and check whether the logic is working if you select D in first dashboard prompt and apply, you should get only A,B,C option in the second prompt.
4. Edit your main dashboard . put Dasgboard prompt1 in one section save.
5. Inside Edit dashboard only click on Dashboard Property (nest to preview button), you can see all the pages.
6. now select your page and click on 'select prompt...' (prompt symbol next to Rename) . Browse your second prompt and click ok,ok.