Even Though John Dislikes Algebra, He Studies Very Hard and Usually

Even though John dislikes algebra, he studies very hard and usually does well on exams.Unfortunately, when he is asked how to do one of the problems that he had answered correctly only one week earlier, he is unable to generate the correct answer.According to Domjan's definitions of learning, what accounts for John's failure?

A) He has not learned algebra.
B) He has forgotten what he learned.
C) He has motivated forgetting.
D) He has dishabituated to that specific problem.