1.  21 25 18 29 33 18
A. 43 18
B. 41 44
C. 37 18
D. 37 41
E. 38 41

Answer: Option D

Explanation:

This is a simple addition series with a random number, 18, interpolated as every third number. In the series, 4 is added to each number except 18, to arrive at the next number.