My stance on the issue is this:
B: It was a restaurant (which was) (at) a long distance from the city.
(which was) = reduced relative
(at) = idiomatically missing prepositionYou may delete the relative pronoun and the be verb when:
1. they are followed by a prepositional phrase.
a long distance from the city = postmodifier --> no preceding comma
It was a distant restaurant.
Student or Learner