Some batches of polio vaccine used around 1960 were contaminated with SV40, a virus that in monkeys causes various cancers. Some researchers now claim that this contamination caused some cases of a certain cancer in humans, mesothelioma. This claim is not undercut by the fact that a very careful survey made in the 1960s of people who had received the contaminated vaccine found no elevated incidence of any cancer, since___________.

A:most cases of mesothelioma are caused by exposure to asbestos

B:in some countries, there was no contamination of the vaccine

C:SV40 is widely used in laboratories to produce cancers in animals

D:mesotheliomas take several decades to develop

E:mesothelioma was somewhat less common in 1960 than it is now