Which of the following sequences is arithmetic?

An arithmetic sequence has the same difference between each pair of consecutive terms. In A, the differences are 9 − 3 = 6, 15 − 9 = 6, 21 − 15 = 6, and 27 − 21 = 6. The other sequences do not have a constant difference, so the sequence 3, 9, 15, 21, 27, … is arithmetic.

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