You are creating an ASP.NET application by using the .NET Framework 3.5. You create a Web form in the application that allows users to provide personal information. You add a DropDownList control to the form to capture the residential country of the user. The default item that the DropDownList control displays is the Select Country option. You need to ensure that users select a valid country other than the default option from the DropDownList control. You also need to ensure that the form is created and maintained by using the minimum amount of development effort. Which validation control should you use?

A.
RangeValidator

B.
CustomValidator

C.
RequiredFieldValidator

D.
RegularExpressionValidator