How to Return a String Var from Web Svc to Client

In ASP.NET 3.5, I am sending form fields to the server via jquery as below.  The server updates the record, then sends a success or failure message back to the client.

But, the client is interpreting a simple string returned to the 'success' function saveResults()  on the client as an object, not a string.  I would expect s to equal "OK", but what I am getting is s.d === "OK", where d is an element of s, apparently.
The web service is returning a simple string.  I've verified it by putting in a break and inspecting the variable 'sResult'.
I have a  jquery function which sends form information to a web service as follows:
.....

  $.ajax({
    type: "POST",
    contentType: "application/json;charset=utf=8",
    url: "Services/ObjectService.asmx/AddCard" ,
    data: ...