答案可以选A也可以选B。即a group of…作主语时，谓语动词可用单数也可用复数。如：

1. A group of women were cackling in a corner. 一群妇女在角落大笑。——出自《剑桥高级英语词典》；谓语用复数were。

2. A group of her friends was waiting for her inside. 她的一群朋友在里面等她。——出自《麦克米伦高阶美语词典》；谓语用单数was。

3. A group of girls was [were] waiting at the school gate. 一群女孩子在校门口等着。——出自《现代英汉词典》外研社；谓语用单数和复数was / were。