selection in <select> tag

Dear friends
I have a problem and I need Your Help
let suppose we have a <SELECT> dropdown list and in which we have 5 options. If in javascript function we pass option no. how can we make that option selected.

<script>
function setIndex(index, selID) {
var obj = document.getElementById(selID);
obj.selectedIndex = index;
}
</script>

(...HTML...)

<span onClick="setIndex(3,'demo');">Set to 3</span>
<select id="demo">
<option value="blah1">blah1</option>
<option value="blah2">blah2</option>
<option value="blah3">blah3</option>
<option value="blah4">blah4</option>
</select>

THANKS BEETLE
IT WORKS
THANKS A LOT