同学们，经过平日的认真学习，你一定积累了好多词汇，想不想检验一下你的词汇量？你知道哪些含有字母n的单词呢？完成下面的字谜吧！一定要保证填入的单词不重复哟！

脑筋急转弯

A ship can contain only fifty persons. Now there are already forty-nine persons in it. At that time， a pregnant woman comes on and boards the ship. The ship sinks. Why？