單項選擇題

You use Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application. 
The application contains three text boxes named txtAddress, txtCity, and txtState. 
You need to ensure that the postal code is automatically set based on the information typed by users in the text boxes.  
You also need to ensure that the complete address is displayed in a TextBlock control. 
Which code fragment should you use?()

A.
B.
C.
D.

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