The correct answer is $\boxed{\text{A. }21, 23}$.
In this sequence, each number is 4 less than the number two places before it. So, the next number in the sequence is 4 less than 23, which is $\boxed{21}$.
Here is a table showing the sequence:
Number | 13 | 29 | 15 | 26 | 17 | 23 | 19 | 21 | 23 -----|------|------|------|------|------|------|------|----- Difference | -4 | 4 | -4 | 4 | -4 | 4 | -4 | 4
Option B is incorrect because 20 is not 4 less than 21.
Option C is incorrect because 20 is not 4 less than 17.
Option D is incorrect because 25 is not 4 less than 23.
Option E is incorrect because 22 is not 4 less than 20.
