Which paired-associate should be the most difficult to learn according to the results of imagery experiments?

Question

Quizplus · Accepted Answer

The paired-associate "quality-necessity" should be the most difficult to learn because it involves abstract concepts, which are harder to visualize compared to more concrete concepts. Imagery experiments have shown that items or pairs that are easier to visualize or imagine tend to be learned and remembered more easily.

Which Paired-Associate Should Be the Most Difficult to Learn According