Can anyone help me please... i'm trying to use class instead of Id but class is not working.

Can anyone help me please... i'm trying to use class instead of Id but class is not working.

<td> <?php $academics=mysql_query("SELECT * FROM tb_academic");
                                        ?>
                    <select onChange="showsub(this.value)" class="required search" id="" name="level"><option>--Select a level--</option>                   
                                                      <?php
                   while($row=mysql_fetch_array($academics))
                   {
                   ?>
            <option id="<?php echo $row['id']; ?>" value="<?php echo $row['academiclevel']; ?>"><?php echo $row['academiclevel']; ?></option>
                
                    <?php
                    }
                    ?></select></td></tr>
                   
                  
                        <script>
function showsub(str)
{
if(str=="")
{
document.getElementById("level").innerHTML="";
return;
}
if (window.XMLHttpRequest)
{
xmlhttp=new XMLHttpRequest();
}
else
{
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("subjecttwo").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","find_subject.php?q="+str,true);
xmlhttp.send();
}


    </script>


                       <td id="subjects">
                      </td>
                       <td id="subjects">
                      </td>
                      <td id="subjects">
                      </td>