ListBox delete all selected items

Hi,

I am trying to delete all the selected items in the listbox.
I am not sure what I am doing wrong.
here is my code:

function DoRemove()
{

var sourceListBox = document.getElementById('ctl00_ContentPlaceHolder1_GetListOfLocation');
var List = sourceListBox.options.length;
 selectedIndex=null;      
 
      if (sourceListBox != null)
      {
        for (var i=0; i < List; i++)
        {
           // if (sourceListBox.options[i].selected)
           if (sourceListBox.Selected[i]) then
           
            {
               // sourceListBox.remove(sourceListBox.options.selectedIndex);
               //sourceListBox.remove(sourceListBox.options[i].selectedIndex);
               //sourceListBox.Items.Delete(i);
            }
        }
    }
}

the error says: 'options[...].selected' is null or not an object.

Thanks,