function answer(id) {
  if(document.getElementById) {
    divO = document.getElementById("o"+id);
    divS = document.getElementById("s"+id);
    testStr = (divO.className=="");
    divO.className = (testStr?"hidden":"");
    divS.src = "images/"+(testStr?"plus":"minus")+".png";
    divS.alt = (testStr?"+":"−");
  }
}