You are developing a Windows Presentation Foundation (WPF) application to play audio files. You add MediaElement mediaElement1 and a Button control named btnPlayAudio to the design surface. The MediaElement co.. audio file. The LoadedBehavior attribute is set to Manual. You add the following code to the main Window. You set the command of the button to MediaCommands.Play. You need to ensure that the application will play the audio file when the button is pressed. What should you add to constructor of the main window ?（）

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