1.  9 12 11 14 13 16 15

A. 14 13
B. 18 21
C. 14 17
D. 12 13
E. 18 17

Answer: Option E

Explanation:

This is a simple alternating addition and subtraction series. First, 3 is added, then 1 is subtracted, then 3 is added, 1 subtracted, and so on.