The system is only able to send emails right now, so a change is necessary. Where? A quick investigation shows that the strategy for sending the greeting is decided in the constructor for the BirthdayGreetingService class following the strategy pattern (https://en.wikipedia.org/?title=Strategy_pattern):
public class BirthdayGreetingService { public BirthdayGreetingService() { messageSender = new EmailMessageSender(); } [...] }