Which of these buildings is most likely to have indoor air pollution

The building from the list provided with the highest likelihood of air pollution is a tightly sealed house with an unvented water heater. The school with leaks will let air exchange happen so that isn't it, and having a carbon monoxide detector only reports existing air pollution it doesn't prevent air pollution, it could have issues but we don't know. The tightly sealed house with the unvented water heater is a near sure thing.