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(); 
  } 
  [...] 
}