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.