问题：

[单选]

You use Microsoft Visual Studio 2010 and Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application. 
You write the following code fragment.  

You need to ensure that the StringMember property of the data-bound object is updated immediately when the user types the TextBox control. 
Which binding expression should you use?（）

A . { Binding Path=StringMember, Mode=TwoWay }
B . { Binding Path=StringMember, NotifyOnSourceUpdated=True }
C . { Binding Path=StringMember, NotifyOnTargetUpdated=True }
D . { Binding Path=StringMember, UpdateSourceTrigger=PropertyChanged }