如何让select中的某几项不能被选择?

Question

<script language="JavaScript">
function chk(obj){ if(obj[obj.selectedIndex].value=="0"){
obj.selectedIndex=0;
}
}
</script>
<select name="ddlShow" id="ddlShow" onChange="chk(this);">
<option  value="-1">asdasd</option>
<option value="1">456546</option>
<option value="0">000000000</option>
</select>

Answer 1

让他的selectedIndex始终等于0就可以～

Answer 2

<script>
function check(form)
{
if (form.ddlShow.value=="0")
form.ddlShow.options[0].selected=1;
}
</script><select name="ddlShow" id="ddlShow" onchange=check(this.form)>
<option  value="-1">asdasd</option>
<option value="1">456546</option>
<option value="0">000000000</option>
</select>

Answer 3

<form name=form2>
<select name="ddlShow" onchange=a()>
<option  value="-1">asdasd</option>
<option value="1">456546</option>
<option value="0">000000000</option>
</select>
</form>
<script>
function a(){
if(document.form2.ddlShow.selectedIndex!=2)
{}
else
document.form2.ddlShow.selectedIndex=0
}
</script>