A certain laboratory is studying the causes of death of a certain population of 10,000 rats. Ten percent of rats in the study population died of liver damage. Ninety percent of the rats that died of liver damage were not exposed to any environmental toxins. Sixty-five percent of the rats whose environments exposed them to low levels of the toxin sulfur dioxide died of liver disorder. Which one of the following best states the main point of the argument above?
a. Environmental and non-environmental causes of liver disease in rats are not mutually exclusive.
b. There is only one cause of fatal liver disease in rats.
c. Environmental toxins are not particularly dangerous to the livers of rats.
d. Sulfur dioxide was not the major cause of death among the rats in the study.
e. Most rats exposed to low levels of sulfur dioxide will not suffer.