C. The average score of the top five students is 90, the total score of the top five students is 450, the average score of the bottom five students is 70, and the total score of the bottom five students is 350, which means that everyone's total score plus the fifth one is 800. From the average score of 8 1, the total score of all people is 729, that is, the score of the fifth place is 7 1, that is, the median is 7 1.