Remove specific Items from ListBox dynamically

public void Populate_ListBoxes(string subj)
{
  var elements = subj.Split(new[] { ',' }, System.StringSplitOptions.RemoveEmptyEntries);
  string lname = null;
  foreach (string items in elements)
  {
    lname = Get_Subjects(items);
    ListItem li = new ListItem();            
    li.Text = lname;
    li.Value = items;
    //ListItem la = listA.Items.FindByValue(items);
    listB.Items.Add(li);
    //compare lname to each of the listItem's text value
    //If comparison successful --> Remove that item from list.
    //I dont know how to compare because listA.Items.Count returns 0
    //How can it return 0 if sqlDataSource is populating listA with some listItems ? ? ??
  }
}

listA.Items.Count

<td>
<asp:ListBox ID="listA" runat="server" DataSourceID="SqlDataSourceSubjects" 
  DataTextField="subject" DataValueField="subjectID" Rows="6" 
  SelectionMode="Multiple" AutoPostBack="True"></asp:ListBox>
<asp:SqlDataSource ID="SqlDataSourceSubjects" runat="server" 
 ConnectionString="<%$ ConnectionStrings:myconnectionstring %>" 
  SelectCommand="SELECT subjectID, subject FROM mast_subject">
</asp:SqlDataSource>
</td>
<td>
<asp:Button ID="btnlistAtolistB" runat="server" 
OnClick="btnlistAtolistB_Click" Text=">>" />
 
<asp:Button ID="btnlistBtolistA" runat="server"                                        OnClick="btnlistBtolistA_Click" Text="<<" />
</td>
<td>
<asp:ListBox ID="listB" runat="server" Rows="6" SelectionMode="Multiple">
</asp:ListBox>
</td>