Custom JS required on dialog for fields Show hide drop down

Hi ALL,

           I'm facing issue on getting dropdown fields for show hide functionality. The logic is 

--> dropdown having three options 

    *no Fields
    *A Fields

    *A and B Fields

1) when user selected "no fields" there will be no fields to show

2) when user selected "A Fields" there will be two fields will present

3) when user selected "A and B Fields" there will be four fields
          *two more fields with additionally with A fields 

please let me know any custom code to acheieve this