The answer, in my opinion, is the second sentence, "The coach who is an expert in the sport explained the revised rules to us". Here, the "Who is an expert", is presumably the nonessential clause.
Maycomb was an old town but it was a tired old town when i first knew it in rainy weather the streets turned to red slop grass grew on the sidewalks and the courthouse sagged in the square.
hope this helped.
