Final answer:
Retroactive interference is the reason why Ryan cannot remember the combo of his oldest lock.
Step-by-step explanation:
Ryan's inability to remember the combo of his oldest lock is a good example of retroactive interference. Retroactive interference occurs when information learned more recently hinders the recall of older information. In this case, Ryan's new lock combo interfered with his ability to remember the combo for his original lock. It is important to note that retroactive interference can affect memory recall in various contexts, not just in the case of a lock combination.