our teacher asked us to write a __articleA.two-thousand-word    B.two-thousand-wordC.two-thousands-words D.two-thousand-words答案选C  但我记得有一道题的答案选three-year-old boy  为什么本题的thousand要加S呢 请给出详细解答 谢谢 什么样的词放在中间需加S?