Answer:
C. An arithmetic sequence with a common difference of 6.
Step-by-step explanation:
A is incorrect because the common difference is not -9 (not constant).
B is incorrect because the common difference is not 3 (not constant).
C is the correct answer because the common difference is 6 (constant).
D is incorrect because this is an arithmetic sequence.
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-9 + 6 = -3
-3 + 6 = 3
3 + 6 = 9
9 + 6 = 15
The common difference is +6 (positive 6).
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