selection from drop down list makes field visible

I have two drop down fields, the select in the first drop down field provides the list of items in the second drop down list. When an item in the second drop down list is selected, I want a hidden field to become visible.

I'm not sure how to write the script on the second drop down list to make the field visible. Here's the script I have, that's not working:

if (this.rawValue == "ACD");

{

     agentID.presence = "hidden";

}