MVVM 跟踪选定的单选按钮

我有一个可观察的集合，它为单选按钮提供值。当用户从一个项目移动到另一个项目时，如果用户之前选择了相应的组单选按钮，我希望自动选择它。

总而言之，这是一个多问题类型的考试，我想在用户从一个问题导航到另一个问题时保持单选按钮处于选中状态。

这是视图：

<ItemsControl ItemsSource="{Binding CurrentQuestion.Choices}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <RadioButton Content="{Binding .}" 
                                 Command="{Binding DataContext.SaveAnswerCommand, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"
                                 CommandParameter="{Binding}"
                                 GroupName="choices"
                                 />
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>

可观察集合：

public static ObservableCollection<ExamModel> Questions
    {
        get
        {
            return _questions;
        }
        set
        {
            _questions = value;
        }
    }
//this is how i keep track of which item is currently active
public ExamModel CurrentQuestion
    {
        get { return _currentQuestion; }
        set
        {
            if (_currentQuestion != value)
            {
                _currentQuestion = value;
                OnPropertyChanged("CurrentQuestion");
            }
        }
    }

模型：

public List<string> Choices
    {
        get 
        {
            if (_choices == null)
                _choices = new List<string>();
            return _choices; 
        }
        set
        {
            if (_choices != value)
            {
                _choices = value;
                OnPropertyChanged("Choices");
            }
        }
    }