8. Sven wrote five different one-digit positive integers on a blackboard. He discovered that none of the sums of two different numbers on the board equalled 10. Which of the following numbers did Sven definitely write on the blackboard?
A 1
B 2
C 3
D 4
E 5
8. E Sven can choose from $\{1,2,3,4,5,6,7,8,9\}$, but can choose at most one from each of the pairs that add to $10:\{1,9\},\{2,8\},\{3,7\},\{4,6\}$. Since this gives him a maximum of 4 integers, he must always pick the digit 5 .