Since the early 1960s, fluoride has been added to many sources of drinking water to prevent cavities in teeth. this is called fluoridation. there are some who say that this is unnecessary and increases the risk of cancer. in 1993, the subcommittee on health effects of ingested fluoride of the national research council, part of the national academy of sciences, conducted an extensive study about the link between fluoridated drinking water and increased cancer risk. the study included data from more than 50 human disease studies and six animal studies. the subcommittee concluded that none of the data showed a link between fluoridated drinking water and cancer. a 1999 report by the cdc supported these findings. the cdc report concluded that studies so far have produced "no credible evidence” of a link between fluoridated drinking water and an increased risk for cancer. according to the article, what valid claim came from the national research council? there is no link between fluoride exposure and increased cancer risk in humans. there is a possible link between fluoride exposure and increased cancer risk in humans. there is a definite link between fluoride exposure and increased cancer risk in humans and animals. there is a possible link between fluoride exposure and increased cancer risk in animals.