To complete the series of numbers, let's analyze the pattern:
The first number in each pair is increasing by 2 (1, 3, 5, 7, 9).
The second number in each pair is decreasing by 1 (10, 9, 8, 7, 6).
Therefore, the next pair in the series would be 11 5.
So, the correct answer is C) 11 5.