second drop down list visible / invisible based on first drop down list choice

I am making a form using livecycle designer.  I want my second drop down list (Conference Room) to either be visible or invisible base on the choice of the first drop down list (Building).  I am new to Javascript and Livecycle.  I found this script online and modify base on my object name, but it does not work.  My script so far is

if (buildingdropdownlist.rawValue == "A") {
  ConferenceRoom.presence = "visible";
} else {
ConferenceRoom.presence = "invisible"; 
}


I am putting this script as a "formready" in the second drop-down list.  Is this where it is suppose to go?  Is the script correct?