Choose dropdown options using JavaScript

Choose dropdown options using JavaScript :
select, options, selectedIndex, selected

<html>
<head>

<script type="text/javascript">
function selectOption(num)
{
    var selObj = document.getElementById('Demo');
    selObj.selectedIndex = num;
}

function alertSelectedIndex(selId)
{
    alert('selectedIndex for ' + selId + ' is: '
     + document.getElementById(selId).selectedIndex);
}
</script>
</head>
 
<body>
<form>
<h3>Choose one option at a time</h3>

<select name="selSS1" id="Demo">
    <option value="val0">sea zero</option>
    <option value="val1">sea one</option>
    <option value="val2">sea two</option>
    <option value="val3">sea three</option>
    <option value="val4">sea four</option>
</select>

<input type="button" value="0" onclick="selectOption(0);" />
<input type="button" value="1" onclick="selectOption(1);" />
<input type="button" value="2" onclick="selectOption(2);" />
<input type="button" value="3" onclick="selectOption(3);" />
<input type="button" value="4" onclick="selectOption(4);" />
<input type="button" value="alert" onclick="alertSelectedIndex('Demo');" />

</form>
</body>
</html>