You use Microsoft .NET Framework 4 to create a Windows Forms application. The application includes a RichTextBox control and a ListBox control. The ListBox control is populated with a list of valid file names. The application allows users to drag a ListBox item to the RichTextBox control. You need to provide a visual indication that a file that is dragged can be dropped on the RichTextBox control. What should you do?（）

• A、 Use a DoubleAnimation class.
• B、 Use a Storyboard animation class.
• C、 Use the DragDropEffects enumeration.
• D、 Use the DragAction enumeration.