Sam's forgetting is most likely due to b. decay.
Sam's forgetting of old friend's telephone numbers is not due to interference caused by communication barriers, massed practice, or encoding failure. His forgetting is explained by the decay theory that claims that memories automatically fade with time.
If Sam's forgetting were an encoding failure, he could not recall the new telephone numbers. But decay forgetting explains his difficulty remembering old numbers belonging to his friends in his former location.
Thus, Sam's forgetting of old friends' numbers in his former location due to his relocation to a new city is caused by decay forgetting.
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