"-¿A ________ vamos a ver esta noche? -A Luis y Gerardo."
I wrote 'quién' because I thought the questioner wouldn't necessarily know that they would be seeing more than one person. That answer was marked wrong. Would we normally use the plural when asking about an unknown number of persons?
EDIT: I just looked at the question again and see that there was a hint (which I managed to miss) about which PLURAL to use, but my question about which we would normally use still stands.