The Data Below Are from Three Separate Studies That Tested

The data below are from three separate studies that tested different treatment options for prostate cancer. Based on these data you might conclude that after 10 years, radioactive seeds are a more effective treatment option than external radiation for prostate cancer. What is a possible confounding factor that could lead to the results below?

A) The men who received external radiation were healthier at the start of the treatment than the men who received radioactive seeds.
B) The men who received external radiation had larger tumors than the men who received radioactive seeds.
C) The men who received radioactive seeds had additional types of cancers whereas the men who received external radiation only had prostate cancer.
D) The men who received radioactive seeds had poor diets compared to the men who received external radiation.