make a selection in a drop down field based on the value of two other field numbers

What should the field's value be if C equals D?

You could use a text field instead of dropdown if you only have two values "A" and "B", and use this as custom calculation script of that text field:

var c = this.getField("C").valueAsString;
var d = this.getField("D").valueAsString;

if (c && d) {
 var numC = Number(c);
 var numD = Number(d);

 if (!isNaN(numC) && !isNaN(numD)) {
  event.value = numC > numD ? "A" : numC < numD ? "B" : "";}} 
else {
 event.value = "";}