is there anyway to insert two default values in place of null in dropdown.

I have a dropdown list where it filled with some rows as null.i am trying to find the nullvalues and try to assign default value in place of null
dropdown looks
ping ertu NULL NULL NULL NULL i written a code for setting default value in dropdown in place of null,but i need two default values in place of null..is there a way..
here is the code..one default value is S_TERM and another value is R_TERM
in place of nulls i want to see these both default values..
aspx file.. <select id="editProduct" runat="server"></select> aspx.cs.. var ss= new List<ListItem>(); foreach (var itm in products) { taxCodes.Add(new ListItem() { Text = itm.Product.ToLower().Equals("null") ? "S_TERM" : itm.Product, Value ...