Suppose That You Hear a Lecture About Memory Improvement, and the Speaker

Suppose that you hear a lecture about memory improvement, and the speaker says, "We must remember the value of desirable difficulties." The speaker is likely to emphasize that

A) academic tests must be difficult enough to differentiate between the students who deserve A's and the students who deserve lower grades.
B) professors who want to assess their academic programs must be sure that the courses are sufficiently difficult.
C) students' motivation will be higher if they are allowed to select the appropriate level of difficulty for the material in a course they are taking.
D) if students test themselves several minutes after learning some material, they will make more errors, which will encourage them to spend more time studying.