有些名词的形式永远是复数形式，其谓语动词既可用单数，也可用复数，这取决于前面的限定词。如：means, series, species.
【例句】
1) A series of attacks by the terrorists occurred on 11th, September 2001, which was a terrible day to all American people and American government.
2) There is no means of learning what has happened in the country, for it has be en completely isolated from the rest of the world by the war.
3) There are four important means of communication: Print,photography,telegraph a nd telephone