题目选项分析

Many office buildings designed to prevent outside air from entering have been shown to have elevated levels of various toxic substances circulating through the air inside, a phenomenon known as sick building syndrome. Yet the air in other office buildings does not have elevated levels of these substances, even though those buildings are the same age as the "sick" buildings and have similar designs and ventilation systems.

Which of the following, if true, most helps to explain why not all office buildings designed to preven't outside air from entering have air that contains elevated levels of toxic substances?

【选项】Most office buildings with sick building syndrome were built between 1950 and 1990.