a pack of后接复数作主语时，谓语动词用单数或复数都可以。如果强调整体，谓语动词用单数形式；如果侧重于组成整体的成员（即强调各个组成整体的个体），谓语动词用复数形式。如：

A pack of reporters were waiting outside. 一群记者等在外面。（朗文）

A pack of journalists was waiting outside the White House. 一帮记者正在白宫外等候。（剑桥）

补充：类似的用法也适用于 a group of, a band of, a bevy of, a block of, a brood of, a bunch of, a circle of, a class of, a cloud of, a clump of, a cluster of, a colony of, a column of, a crop of, a crowd of, a drove of, a flock of, a gang of, a herd of, a host of, a line of, a pack of, a set of, a fleet of, a sprinkle of, a field of, a party of, a pile of, a rank of, a regiment of, a row of, a school of, a swarm of, a team of, a tribe of, a troop of, a bundle of, a slew of 等表达。如：

A group of girls was / were waiting at the school gate. 一群女孩子在校门口等着。

A team of experts has / have been called in to investigate. 一个专家小组已应召来进行调查。

A bundle of books was / were crammed into the back of the car. 一捆书被塞进了车子的后部。

A bunch of girls was / were sitting on the grass. 一群女孩坐在草地上。

A gang of robbers was / were involved in a shoot-out with police yesterday. 一伙抢劫者参与了昨天与警察的一次枪战。

A flock of sheep has / have escaped onto the road. 一群羊跑到路上去了。

A herd of cows was / were descending into the valley. 一群牛正走下山谷。

A team of gardeners tends / tend the grounds. 一组园艺工人照看这块场地。

A crowd of onlookers was / were gathering to see the film being made. 一大群旁观者围拢来看拍摄电彩。

A row of police cars was / were blocking the road. 一排警车堵在路上。