You create a Microsoft ASP.NET application by using the Microsoft .NET Framework
version 3.5.You create a login Web form by using the following code fragment.
<asp:ScriptManager ID=”ScriptManager1″ runat=”server” />
<asp:TextBox runat=”server” ID=”txtUser” Width=”200px” />
<asp:TextBox runat=”server” ID=”txtPassword” Width=”200px” />
<asp:Button runat=”server” ID=”btnLogin” Text=”Login” OnClientClick=”login(); return
false;” />
When a user clicks the btnLogin Button control, the login() client-side script is called to
authenticate the user.The credentials provided in the TextBox controls are used to call the
client-side script. You also add the following client-script code fragment in the Web form.
(Line numbers are included for reference only.)
01 <script type=”text/javascript”>
02 function login() {

03 var username = $get(‘txtUser’).value;
04 var password = $get(‘txtPassword’).value;
05
06 // authentication logic.
07 }
08 function onLoginCompleted(validCredentials, userContext,
09 methodName)
10 {
11 // notify user on authentication result.
12 }
13
14 function onLoginFailed(error, userContext, methodName)
15 {
16 // notify user on authentication exception.
17 }
18 </script>
The ASP.NET application is configured to use Forms Authentication. The ASP.NET AJAX
authentication service is activated in the Web.config file.You need to ensure that the
following workflow is maintained:
On successful authentication, the onLoginCompleted client-script function is called
to notify the user.
On failure of authentication, the onLoginFailed client-script function is called to
display an error message.
Which code segment should you insert at line 06?

A.
var auth = Sys.Services.AuthenticationService;
auth.login(username, password, false, null, null,onLoginCompleted, onLoginFailed, null);

B.
var auth = Sys.Services.AuthenticationService;
auth.set_defaultFailedCallback(onLoginFailed);
var validCredentials = auth.login(username, password, false, null, null, null, null, null);
if (validCredentials) onLoginCompleted(true, null, null); else
onLoginCompleted(false, null, null);

C.
var auth = Sys.Services.AuthenticationService;
auth.set_defaultLoginCompletedCallback(onLoginCompleted);
try {
auth.login(username, password, false, null, null, null, null, null);
}
catch (err) {
onLoginFailed(err, null, null);
}

D.
var auth = Sys.Services.AuthenticationService;
try {
var validCredentials = auth.login(username, password, false, null, null, null, null, null); if
(validCredentials) onLoginCompleted(true, null, null);
else
onLoginCompleted(false, null, null);
}
catch (err) {
onLoginFailed(err, null, null);
}

Explanation:
Sys.Services.AuthenticationService Class: http://msdn.microsoft.com/en-us/library/bb310861.aspx