Insert the missing number.
3, 7, 6, 5, 9, 3, 12, 1, 15, (....) Pick one.
A) 3
B) 7
C) -1
D) 8
E) -45
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.   Answer: -1) -1
<result>C</result>
Insert the missing number.
3, 7, 6, 5, 9, 3, 12, 1, 15, (....)