In a study it was found that eating vegetables rich in beta-carotene could reduce cancer. But there can be other factors like high fiber diet, which could also be responsible for this. This is due to

High fiber diet is associated with vegetables rich in beta-carotene. Also, high fiber diet is associated with decreased incidences of ceain cancers especially colonic cancers. Now, since the high-fiber diet is present with both the cause and the effect independently and is distributed unevenly in the case and the control group, it can be called a confounding factor. Also, it can be due to multifactorial causation, if both carotene and high fiber diet are associated with reduced cancer incidence. Since we have to select only one option, and from other studies we do know that vitamin doesn't decrease the risk of cancer, the best option will be confounding factor.