Filtro Selects em JS

<html>

<head>

<script type="text/javascript">

function mudar(obj) {
sumirTudo();
if(obj.value != '') aparecer('sel_' + obj.value);
}

function sumirTudo() {
document.getElementById('sel_1').style.display = 'none';
document.getElementById('sel_2').style.display = 'none';
document.getElementById('sel_3').style.display = 'none';
}

function aparecer(id) {
document.getElementById(id).style.display = 'block';
}

</script>

</head>

<body onload="sumirTudo();">

<select onchange="mudar(this)">
<option value=""></option>
<option value="1">a</option>
<option value="2">b</option>
<option value="3">c</option>
</select>

<select id="sel_1">
<option value="1">a1</option>
<option value="2">b1</option>
<option value="3">c1</option>
</select>

<select id="sel_2">
<option value="1">a2</option>
<option value="2">b2</option>
<option value="3">c2</option>
</select>

<select id="sel_3">
<option value="1">a3</option>
<option value="2">b3</option>
<option value="3">c3</option>
</select>

</body>

</html>