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.
The answer is C. Suspenseful and terrified. Why? because the writer is explaining it in a scary way of how she grabbed him by the shirt and shook him so hard that his teeth could have fallen out.
After “because,” you always want to put an explanation.
