Please answer the following question by reasoning step-by-step.
Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next?\nOptions:\n(A) 15\n(B) 14\n(C) 17\n(D) 18\n(E) 19

(A)
In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.

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The answer is (A)