Insert the missing number: 14, 9, 5, 21, 8, 13, 28, 9, _____?

None of these

The pattern of the sequence appears to be a alternating addition and subtraction of numbers:

14 - 5 = 9

9 - 4 = 5

5 + 16 = 21

21 - 13 = 8

8 + 5 = 13

13 + 15 = 28

28 - 19 = 9

So, the next number in the sequence would be:

9 + 10 = 19