Traditional Logic II Answer Key Error?

[simtski]

I was checking over my work in Traditional Logic II, Exercises for Chapter 4, Day 2, when I encountered the question (#13), "Which two sets of statements contradict one another?" I answered, "A & O, E & I." The answer key, however, says (paraphrased),"A & O, E & O." Is this an error in the material, or do I not understand contradiction?