当“a line of+复数名词”用作主语时,其谓语动词可用单数(视为整体)也可用复数(侧重个体)。如:
A line of low hills was seen in the distance.(《张道真英语用法》)
A ragged (= not straight) line of people were waiting at the bus stop.(《剑桥高级英语词典》第3版)
a line of ... 作主语谓语用单数。例如:
A line of people or things is a number of them arranged one behind the other or side by side.(《Collins COBUILD Advanced Learner's English - Chinese Dictionary 》line 词条)