[求助]关于which, clause, 和单复数的问题

"ChaseDream TOEFL语法大全"中收的19891T语法题:

Barges which carrier most of the heavy freight on rivers and canals are usually propelled by towing.

同一文档中随附的答案如下：

答案:A
测试点:词性
分析：carrier是名词，而此处需要谓语动词。应为carries.

我觉得Barges是复数名词，那么正确的写法应该是carry，而不是carries

参照了一些英文句子，也印证了这个想法

在此向各位大虾求证，是这个语法大全中的笔误？还是我的理解有问题？

Any comment is appreciated / Thanks in advance.

Barges /which carrier most of the heavy freight on rivers and canals/ are usually propelled by towing.

后面都用are了，应该用carry 啊。