我的XAML出了什么问题？

    <GroupBox x:Name="groupBox" Header="Operating System" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Height="74" Width="280">
        <RadioButton GroupName="Os" Content="Windows XP" IsChecked="True"/>
        <RadioButton GroupName="Os" Content="Windows Vista" />
        <RadioButton GroupName="Os" Content="Windows 7" />

    </GroupBox>
    <GroupBox x:Name="groupBox" Header="Operati