Although exposure to asbestos is the primary cause of mesothelioma, a slow-developing cancer, researchers believe that infection by the SV40 virus is a contributing cause, since in the United States 60 percent of tissue samples from mesotheliomas, but none from healthy tissue, contain SV40. SV40 is a monkey virus; however, in 1960 some polio vaccine was contaminated with the virus. Researchers hypothesize that this vaccine was the source of the virus found in mesotheliomas decades later. Which of the following, if true, most strongly supports the researchers' hypothesis? ['In Finland, where the polio vaccine was never contaminated, samples from mesotheliomas do not contain SV40.', 'Changes in the technique of manufacturing the vaccine now prevent contamination with SV40.', 'Recently discovered samples of the vaccine dating from 1960 still show traces of the virus.', 'SV40 is widely used as a research tool in cancer laboratories.']