What Does NOT Explain Why Indoor Air Pollution May Pose

What does NOT explain why indoor air pollution may pose a bigger threat than outdoor air pollution in some locations?

A) We breathe more air indoors because we spend so much time in homes,schools,and/or the workplace.
B) Items in our home such as paint,cleaners,and furniture release VOCs,which can cause health problems.
C) Outdoor pollutants such as radon can also find their way into our buildings and concentrate in enclosed areas.
D) Secondary pollutants form and accumulate more easily in indoor environments.
E) All of these answers explain why indoor air pollution may pose a bigger threat than outdoor air pollution.