1] our house was once a stop on the underground railroad. [2] the underground railroad were a network of hiding places. [3] the cellar beneath our house has a secret door that was installed to trick slave catchers. [4] living in a piece of american history is pretty interesting. which sentence has a subject-verb agreement error?

Based on the sentences given, it is sentence 2 that has an error on subject-verb agreement. Notice that the subject is "Underground Railroad" and this is singular. Therefore, the verb that it should take is singular too. The correct verb to be used should be "was" and not "were". So the correct sentence is this: The Underground Railroad was a network of hiding places.

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