One study found that 70% of children surveyed in the 1970 had ever had worm teeth, compared to only 50% of the children surveyed in the 1985 survey. The researchers concluded that between 1970 and 1985 Over the course of this year, the rate of dental disease among children has decreased. If true, the one thing that would weaken the researchers' conclusions above is:. ['Children surveyed come from families with different income backgrounds.', 'Worm tooth is the most common type of dental disease that children may have.', 'Children surveyed in 1985 are younger than children surveyed in 1970.', 'The children surveyed were selected from students of teachers who collaborated with these researchers.']