1.  3, 7, 6, 5, 9, 3, 12, 1, 15, (....)
A. 18
B. 13
C. -1
D. 3

Answer: Option C

Explanation:

There are two series, beginning respectively with 3 and 7. In one 3 is added and in another 2 is subtracted.

The next number is 1 - 2 = -1.