Dropdown array validation

i want to validate that a selection is made when the form is submitted. the tricky part is i also have an option where the user can add more rows so that they can insert more than 1 record at a time. how do i get the js to validate?

this is my html and js notice the name of the select is ‘cid

  <td bgcolor="#666666"><SELECT name='cid[]' class="one_pixel_border">
           <option selected value="">Select One</option>

               $cQuery = mysql_query("SELECT cid, category FROM category");  

           while ($sRow = mysql_fetch_row($cQuery))

                                                   print ("<OPTION value='$sRow[0]'>$sRow[1]</OPTION>");
          </SELECT> </td>

  <script language="javascript1.2" type="text/javascript">

function valbutton(thisform)

    if (thisform.cid.selectedIndex < 1)
    alert("Please select a category.");
    return false;



By javascript we can handle drop down array list by this Funtion
function addOption(selectbox,text,value )
var optn = document.createElement(“OPTION”);
optn.text = text;
optn.value = value;