You are creating an ASP.NET application by using the .NET Framework 3.5.

You use the built-in ASP.NET Web server controls in all the Web forms of the application.

You access and modify the properties of all Web server controls in the code-behind class of the Web forms.

You need to add custom client-side and AJAX behavior to the ASP.NET Web server controls.

What should you do?

A.
Add an UpdatePanel control in the Web forms for each built-in ASP.NET Web server control.
Place the corresponding ASP.NET Web server controls inside the UpdatePanel controls.
Create a custom extender control for each built-in ASP.NET Web server control.

B.
Add the extender controls along with the ASP.NET Web server controls in the Web forms.
Create a Web UserControl control for each built-in ASP.NET Web server control. Encapsulate the corresponding ASP.NET Web server control and the client-side and AJAX behavior code in the UserControl control.

C.
Replace each built-in ASP.NET Web server control with the UserControl control in the Web forms. Create a custom Web server control for each built-in ASP.NET Web server control.
Add the corresponding ASP.NET Web server control as a child control, and encapsulate the client- side and AJAX behavior code in the custom Web server control.

D.
Replace each built-in ASP.NET Web server control with the custom Web server control in the Web forms.