21, 9, 21, 11, 21, 13, 21, ?
Choose one:	A) 11	B) 17	C) 19	D) 15	E) 20
D   15   In this alternating repetition series, the number 21 is interpolated. If you exclude 21, the series increases by 2, beginning with the number 9.
<result>D</result>
21, 9, 21, 11, 21, 13, 21, ?