Acrobat Pro How to set a check box with export value as read only depending on a drop down selection

+++UPDATE

I just wanted to add to the answer to this question :

•  I want the end user to only tick one box at the time. Similar to a radio button function but I don't want to use a radio button.

Here is what work for me:

var g = event.value;
var f =  this.getField("myDropdown");

if ((g =="A") || (g =="B") || (g =="C"))  {

f.checkThisBox(2, true);
 
this.getField(f.name+".2").fillColor =  ["G",0.7]; 

f.readonly = true;


}  else {


f.checkThisBox(2, false);

f.readonly = false;

f.fillColor = ["RGB",0.874,0.874,1];

}

Selecting any any item that is not A, B or C allows to continue to check  the boxes again. 

This part of the script this.getField(f.name+".2").   doesn't work with the readonly attribute though

You will get the last checkbox in color gray, checked automatically upon the A, B, C selections from the dropdown list, but then  it makes the enntire parent group of checkboxes read-only. I am not sure if you wanted to have the ability to continue to check other boxes when the third one was made read-only.

 I haven't been able to figure that one out yet. But this works as far as one selection at a time just like radio buttons would  behave.

Use this code as "Custom Calculation Script" of your dropdown field.

var a = event.value;
var x = this.getField("Z");
if(a == "A" || a == "B" || a == "C"){
x.readonly = true
x.fillColor = ["G", 0.7];
}
if(a == "D"){
x.readonly = false
x.fillColor = color.white;
}

 Don't forget to rename fields to your field names and change color white if you want any other color.