You use Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application. The application has a window named MainWindow that has a StackPanel control named sp as the root element. 
You want to create a Button control that contains a TextBlock control with the "Save" Text property. 
You need to create the control dynamically and add the control to sp. 
Which code segment should you write in the constructor of the MainWindow class?（）