What is a prepositional case?

Tia and lin had been best friends since kindergarten. both avid soccer players, the girls decided to try out for the high school team together. unfortunately, there was just one open spot, so only one of the girls would be chosen. they both worked hard, and on tryout day, they did their best. when the team roster came out, lin was excited to see that she had made the team but sad that tia had not. tia was happy for her friend and vowed to work harder so she would make the team the next year. which sentence is the exposition of the passage