HI Feroz,
you can use onchange client script wherein you can see the value selevted in field A and depending on that you can add/remove options in field B(choice List Variable)
Use these functions for the same
g_form.addOption('<Variable Name>','<value>','<display value>'); // To add options to the dropdown
g_form.clearOptions('<variable name>'); //To remove all values from the dropdown
g_form.removeOption('<Variable Name>', '<value>'); // To Remove 1 option from the dropdown
g_form.setValue('<Variable Name>','<value>'); // To select by default any one of the options.
Hope this helps.
-Anurag
