a group of students是单数还是复数

　　The college is playing host to a group of visiting Russian scientists.

　　学院正在接待一批来访的俄罗斯科学家。

　　The consultants were a group of men in sharp suits.

　　顾问都是些衣着入时的男人。

　　A group of downcast men stood waiting for food.

　　一群人垂头丧气地站在那儿等着吃饭。