I am studying English grammar.
This is a sentence written about TV in my English textbook.

"A hundred years ago people would not have believed their ears or eyes if someone had said that by turning the dial of a machine they could hear a concert or see a ballet (which [were, was]) given hundreds of miles away."

About part in parentheses, which is correct? Were or was.
The answer of my textbook is "which were".
However, I though "was" was correct.

Please someone help!