What is the author's purpose in using the structure element of flashback in a story

Passage: mrs. smith’s sixth-grade class filed onto the grassy field behind the middle school after lunch. as she exited the building, chelsea looked around for her closest friend, brittany. she spotted her by the swings, talking to their classmate brian. 2 “brian thinks he’s the fastest person in mrs. smith’s class,” brittany told her when she had joined them. it was obvious that brittany didn’t agree with brian. 3 “i guarantee i can beat anyone in this class to a race,” brian said, tugging on the scarlet baseball cap that he wore each and every day. “in fact, i bet i can run faster than the seventh, eighth, and even ninth graders, too! ” 4chelsea rolled her eyes, but wasn’t astonished by his declaration. brian was typically overconfident; he was always challenging other students and expecting to win. according to brian, he was so talented that he could hit more home runs than children on the baseball team and even could repeatedly make more baskets than the boys and girls on the basketball team. 5 “who dares to challenge me? ” brian shouted, directing his question to every student on the playground. the students looked around, but no one volunteered. 6 “i’ll race you,” chelsea said, breaking the awkward silence. she knew she wasn’t the fastest person on the playground, but if she possessed the determination necessary to win, perhaps she could complete the race first. 7 “first person around the field wins,” said brian, stretching his legs and jumping up and down to ready himself for the race. 8they approached the starting line, and brittany counted to three. 9 “see you at the finish line! ” brian exclaimed and left chelsea in the dust. 10brian soon disappeared around the first turn. chelsea took a deep breath and continued to run at a quick pace. the sun was warm and she soon grew tired, but she didn’t stop. ahead, brian approached the finish line and as he began to celebrate, she watched him trip over a small mound of dirt and fall to the ground. 11as chelsea reached the finish line, she noticed that brian was still seated on the ground, rubbing his injured knee. instead of running past him and winning the race, chelsea stooped down to him stand. he leaned on her shoulder, and they completed the race together. 12 “you could've won,” brian said as their friends came out to congratulate them. 13chelsea smiled. “friends are more important than winning,” she said. 14brian smiled at her. “my thoughts exactly,” he said question: which action in the story advance the plot the most? a) when chelsea accepts brian's challenge to race him b) when chelsea rolled her eyes at irritation at brian's pride c) when chelsea reached the finish line and brian finish the race d) when brittany told chelsea that brian thinks he's the fastest in the class