10 Brian 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.

11 As 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 help him stand. He leaned on her shoulder, and they completed the race together.
Chelsea felt tired during the race because she

A) grew warm from the sun.
B) ran faster than Brian did.
C) ran further than Brian did.
D) didn't practice for the race.