If Nikki's fall sport is running, then which one of the following statements must be true?

i'm confused on this because, the two options for fall N and O in my game setup is R/M and M/R. When you place R for N, of course M will have to go in O. However, Since the students must play 4 sports and the only options left in my setup for them are K/M, Otto's summer sport would have to be K. Since N's summer sport can't also be K, the only other option is M, which would only leave the option K for N's spring sport. Why is this not the answer?