wpf中一组radiobutton为什么不能设默认选项？解决方法

wpf中一组radiobutton为什么不能设默认选项？

C# code

<RadioButton Content="男" IsChecked="True" Tag="1"  Height="16" HorizontalAlignment="Left" Margin="16,16,0,0" GroupName="Sex" VerticalAlignment="Top" Checked="RadioButton_Checked" />            <RadioButton Content="女"  Tag="2" Height="16" HorizontalAlignment="Left" Margin="103,16,0,0" VerticalAlignment="Top" GroupName="Sex" Grid.Column="1" Grid.Row="4" Checked="RadioButton_Checked" />