Which of the following public charities has unrelated business income?

$$ Wear It Proud, Inc., which operates a retail store that sells donated merchandise.

$$ Vets for Pets, Inc., which operates vending machines on$-$site for the use of employees.

O Spirit Boosters, Inc., which charges for parking during sporting events that take place at an unrelated nearby stadium.

$$ Operation Wilderness, Inc., which operates a summer day camp that is staffed entirely by volunteers.