Any help understanding why the answer is A and not B in this Pardot Certification Sample questions is much appreciated!

Quiz Question:

LenoxSoft would like to implement a re-engagement program for prospects who are no longer active on their website. How should a Pardot marketer design an engagement program focused on providing exclusive offers and content to re-engage prospects?  Choose one answer

1. Create an automation rule that adds prospects to a list with the criteria Prospect time :: last activity :: greater than :: 180 days ago. Add the list as the recipient list to an engagement program that leads re-engaged prospects to a promotional offer.
2. Create a dynamic list that matches prospects with the criteria Prospect time :: last activity :: greater than :: 180 days ago. Add the list as the recipient list to an engagement program that leads re-engaged prospects to a promotional offer.
3. Create a segmentation list that matches prospects with the criteria Prospect time :: last activity :: greater than :: 180 days ago. Schedule a list email that includes a promotional offer to autosend every Friday going forward to re-engage prospects.
4. Create a dynamic list that matches prospects with the criteria Prospect time :: last activity :: greater than :: 180 days ago. Schedule a list email that includes a promotional offer to autosend every Friday going forward to re-engage prospects.

The Answer is A.

I chose B because I thought that an automation rule would be an extra step, and that it would be a simpler process to just use a dynamic list with engagement studio. As prospects meet the criteria in the dynamic list, they will be added to the list and then the engagement program.  What is the purpose of the automation rule?