Which of the Following Was Mentioned as One of the Factors

Which of the following was mentioned as one of the factors that could possibly account for the substantial and rapid forgetting that occurred to Hermann Ebbinghaus?

A) His use of acronyms was not well suited to the memory tasks he set for himself.
B) He learned so many lists that it must have been difficult to distinguish between them.
C) He failed to use maintenance rehearsal.
D) The material he learned required excessive deep processing.