dvojity select

<style type="text/css">
      select {display:none;}
      #volba0 {display:block;}
      fieldset {border:none;}
    </style>
<script type="text/javascript">
function vypis(volba)
{
for (i=1; i <= 3; i++)
{
document.getElementById('volba'+i).style.display='none';
}
document.getElementById(volba).style.display='block';
}
</script>
<select size="1" id="volba0" onchange="vypis(this.options[this.selectedIndex].value);">
          <option value="volba0">
          Značka
          </option>
          <option value="volba1">
          Škoda
          </option>

          <option value="volba2">
          Audi
          </option>
          <option value="volba3">
          BMW
          </option>
        </select>
        <select  size="1" id="volba1">
          <option>
          Škoda1
          </option>

          <option>
          Škoda2
          </option>
        </select>
        <select  size="1" id="volba2">
          <option>
          Audi1
          </option>
          <option>
          Audi2
          </option>

        </select>
        <select  size="1" id="volba3">
          <option>
          BMW1
          </option>
          <option>
          BMW2
          </option>
        </select>

<form action="/xxx.html" method="POST" name="f">
<script type="text/javascript">
		kat = new Array(3);
				kat[1] = new Array();
				kat[2] = new Array();
				kat[3] = new Array();
				kat[1][1] = 'aaa 1';
				kat[1][2] = 'aaa 2';
				kat[2][3] = 'bbb 1';
				kat[2][4] = 'bbb 2';
				kat[2][5] = 'bbb 3';
				kat[3][6] = 'ccc 1';
				kat[3][7] = 'ccc 2';
				kat[3][8] = 'ccc 3';
				kat[3][9] = 'ccc 4';
				kat[3][10] = 'ccc 5';
				
					function change_kat(val)
	{
		document.f.subkat.options.length = 0;
		pocet = 0;
		for (i=0;i<kat[val].length;i++) {
			if (typeof kat[val][i] != "undefined") {
				pocet++;
			}
		}
		if (pocet>0) {
			document.f.subkat.options[0] = new Option('zde upřesněte','');
			j = 1;
			for (i=0;i<kat[val].length;i++) {
				if (typeof kat[val][i] != "undefined") {
					document.f.subkat.options[j] = new Option(kat[val][i],i);
					j++;
				}
			}
			document.f.subkat.options[j] = new Option('není tu vhodná podkategorie','0');
		} else {
			document.f.subkat.options[0] = new Option('není podkategorie','0');
		}
	}

	</script>
   	<select name="kat" onChange="javascript:change_kat(this.value)">
		  <option value="">Nejdříve vyberte</option>
      <option value="1">aaa</option>
      <option value="2">bbb</option>
      <option value="3">ccc</option>
    </select>
   	<select name="subkat">
		  <option value="" style="width: 250pt">zde upřesněte</option>
    </select>
</form>