[JavaScript] select form selected

HTML Code
<form action='test.php method='post' name='test_form'>
<select name="operator">
   <option value="+">+</option>
   <option value="-">-</option>
   <option value="*">*</option>
   <option value="/">/</option>
   <option value="?">바코드</option>
</select>

JavaScript Code
<script>
document.test_form.operator[2].selected = true;
</script>

Output
Select Box가 *로 선택되어져 있다.