Look at this series: 21, 9, 21, 11, 21, 13
What number should come next?
I. 14
II. 15
III. 21
IV. 23
Question: 以下哪项是正确的？
Options:
A. 以上各项均不成立, B. 只有 III, C. 仅 IV, D. 仅 II
Answer: B