1.  8 11 21 15 18 21 22

A. 25 18
B. 25 21
C. 25 29
D. 24 21
E. 22 26

Answer: Option B

Explanation:

This is an alternating addition series, with a random number, 21, interpolated as every third number. The addition series alternates between adding 3 and adding 4. The number 21 appears after each number arrived at by adding 3.