Recent epidemiological studies have shown that the residents of Greenborough County have substantially higher rates of pancreatic and stomach cancer than do the residents of the state as a whole. Researchers have concluded that the high rates of these cancers are due to the presence in the county of a large petrochemical facility that manufactures chemicals known to be carcinogenic.
Which of the following, if true, most weakens the researchers’ conclusion ？（）

A. The county’s residents have the highest rate in the state of smoking, a habit that has been conclusively linked with higher rates of several types of cancer.

B. The residents of the county get their water from springs that could not have been contaminated by the petrochemical plant implicated by the researchers.

C. The petrochemical plant has been found to be in violation of EPA emissions standards five times in the last two years.

D. Residents of Greenborough County, like people throughout the state, are fond of spicy and fatty foods that have been linked to a number of health problems.

E. There are no known medical treatments that can permanently halt the development of certain forms of pancreatic cancer.