Drop down Box Selected

<select id="mySelect" multiple="multiple">
<option value="1">First</option>
<option value="2">Second</option>
<option value="3">Third</option>
<option value="4">Fourth</option>
</select>

<script type="text/javascript">
$(document).ready(function() {
if (!$("#mySelect option:selected").length)
$("#mySelect option[value='3']").attr('selected', 'selected');
});
</script>