Which of these buildings is most likely to have indoor air pollution?
a. a school with leaks that allow water and heat to be wasted.
b. an office building without a carbon monoxide detector.
c. a tightly sealed house with an unvented water heater.

When heater is on for long time without having ventilation then carbondioxide will be converted into carbon monoxide and causes air pollution.
So according to above explanation,
C. A tightly sealed house with an unvented water heater, is the correct answer.

That’s what i was going to say.