You are developing a Windows Presentation Foundation (WPF) application. You build a dictionary that implements INotifyPropertyChanged. You want to use an indexer to bind to the UI. The relevant…

You are developing a Windows Presentation Foundation (WPF) application. You build a dictionary that implements INotifyPropertyChanged. You want to use an indexer to bind to the UI. The relevant implementation of the dictionary is as follows. (Line numbers are included for reference only.) You need to ensure that the PropertyChanged event is triggered so that the UI is updated if you modify the collection using the indexer. Which code should you insert at line 17?