You use Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application. You write the following code fragment. You create an event handler named StackPanel_PreviewTextInput. You also have a collection of strings named Keywords. You need to ensure that TxtBoxA and TxtBoxB do not contain any of the strings in the Keywords collections. Which code segment should you use?（）

• A、
• B、
• C、
• D、