You use Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application. 
You write the following code segment. (Line numbers are included for reference only.) 

the TextBox control is data-bound to an instance of the Contact class.  
You need to ensure that the Contact class contains a business rule to ensure that the ContactName property is not empty or NULL.
You also need to ensure that the TextBox control validates the input data. 
Which two actions should you perform?（）

You use Microsoft Visual Studio 2010 and Microsoft . 
NET Framework 4 to create a Windows Presentation Foundation (WPF) application. 
You create a WPF window in the application.  
You add the following code segment to the application. 
public class ViewModel 
{ 
public CollectionView Data { get; set; } 
}
public class BusinessObject 
{ 
public string Name { get; set; } 
}
The DataContext property of the window is set to an instance of the ViewModel class.  
The Data property of the ViewModel instance is initialized with a collection of BusinessObject objects. 
You add a TextBox control to the Window. 
You need to bind the Text property of the TextBox control to the Name property of the current item of the CollectionView of the DataContext object.  
You also need to ensure that when a binding error occurs, the Text property of the TextBox control is set to N/A . 
Which binding expression should you use?（）