Insert the missing number.
3, 9, 6, 6, 9, 3, 12, 0, 15, (....)  Choose the correct choice:
A) 3
B) 7
C) -3
D) 8
E) -45
Explanation:   There are two series, beginning respectively with 3 and 9. In one 3 is added and in another 3 is subtracted.   The next number is 0 - 3 = -3.   Answer: -3) -3
<result>C</result>
Insert the missing number.
3, 9, 6, 6, 9, 3, 12, 0, 15, (....)