// JavaScript Document

   function a(obj){
     var _selectedvalue = obj.value;
  if(_selectedvalue==''){
     _selectedvalue = '-1';
  }

  for(i=1;i<=3;i++){
    var divobj = document.getElementById('tb'+i);
    if(parseInt(_selectedvalue)==i){
       divobj.style.display = 'block';
    }else{
       divobj.style.display = 'none';
    }
  }
   }