Researchers wanted to know if secondhand tobacco smoke is a significant factor in contracting lung diseases. They conducted a study comparing the health of a group of nonsmoking spouses of smokers with that of a group of nonsmoking spouses of nonsmokers. It was discovered that the spouses of smokers were significantly more susceptible to lung diseases than were the spouses of nonsmokers. The researchers concluded that secondhand tobacco smoke is indeed a factor in the development of lung diseases. Which one of the following, if true, most calls into question the researchers' conclusion? ['Significantly more of the spouses of smokers were raised by smokers than were the spouses of nonsmokers.', 'The group of spouses of smokers was selected from a heavily industrialized urban area, while the group of spouses of nonsmokers was chosen from a rural area.', 'The members of the group of spouses of smokers were on average several months younger than the members of the group of spouses of nonsmokers.', 'The spouses of nonsmokers are no more likely to be former smokers than are the spouses of smokers.']