>> if I choose 4 then the textbox should display "n/a"
You want to create a rule to set the textbox value
(1) From the Home tab, select "Manage Rules", this will open the Rules pane
(2) Left-click once on your textbox
(3) In the Rules pane, select New -> Action to add a new action rule
Condition: DD2 is equal to 4
Run These Actions -> Add -> Set a field's value
Field: DD2
Value: n/a
(4) Add similar rules for when DD1 is equal to c and DD1 is equal to d