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.