21, 9, 21, 11, 21, 13, 21, ?  Select.
A) 15	B) 17	C) 23	D) 25	E) 27
In this alternating repetition series, the number 21 is interpolated. If you exclude 21, the series increases by 2, beginning with the number 9.   15nswer : 15.
<result>A</result>
21, 9, 21, 11, 21, 13, 21, ?