Ciao scusatemi il disturbo potete darmi un aiuto potete dirmi perchè onchange non mi fa eseguire il javascript Grazie

codice:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8" />
    <title>Prenotazione</title>

    <script language="javascript">
    
    function scegli(){ 
     if(document.getElementById("partenza").options[4].selected == "a") 
      {
       
       document.getElementById("arriv").style.display="none"; 
       document.getElementById("arrivo1").style.display="block";
       document.getElementById("arrivo").style.display="none";
       
      }
        else {
        document.getElementById("arriv").style.display="none";
        document.getElementById("arrivo").style.display="none";
        document.getElementById("arrivo1").style.display="block";
        
        }
       
    }
    
    </script>
  </head>
  <body> Partenza
    <form name="form1" id="form1" method="post" action="prenotazioni.php">
      <div id="partenza" style="position:absolute; " >
        <select name="partenza" id="partenza" onchange="javascript:scegli()">
          <option value="0"> Scegliere una partenza </option>
          <option value="1">Altamura</option>
          <option value="2">Avetrana</option>
          <option value="a">Banzi</option>
          <option value="4" >Bari</option>
</select>
      </div>
      <div id="text" style="position: absolute; left: 250px; top: 7px; height: 12px;">
        ARRIVO </div>
      <div id="arriv" style="position: absolute; left: 250px; top: height: 12px; display: block; ">
        <select name="arrivos3" id="arrivos3">
          <option value="0"> Scegliere un arrivo </option>
        </select>
      </div>
      <div id="arrivo1" style="position: absolute; left: 250px; top: height: 12px; display: none; ">
        <select name="arrivos1" id="arrivos1">
          <option value="0"> Scegliere un arrivo </option>
          <option value="1">Altamura</option>
          <option value="2">Avetrana</option>
          <option value="3">Banzi</option>
          <option value="4">Bari</option>
        </select>
      </div>
      <div id="arrivo" style="position: absolute; left: 250px; top: height: 12px; display: none; ">
        <select name="arrivos" id="arrivos">
          <option value="0"> Scegliere un arrivo </option>
          <option value="a">Bologna</option>
          <option value="b">Milano</option>
          <option value="c">Novara</option>
          <option value="d">San Donato Milanese</option>
          <option value="e">Torino</option>
        </select>
      </div>

 <input type="submit" value="CERCA" name="CERCA" onclick="" style="position:absolute; top:60px;" />
    </form>
    
  </body>
</html>