Which situation can be modeled by a function?

A. Raquel can spend $15 on 1 shirt, $30 on 2 shirts,or $30 on 3 shirts.

B. Bonnie can spend $10 on a shirt, $20 on 2 shirts, and $20 on 3 shirts.

C. Maria can spend $15 on 1 shirt, $15 on 2 shirts, or $30 on 3 shirts.

D. Natalie can spend $15 on 1 shirt, $30 on 2 shirts, or $45 on 3 shirts.