Consider a spontaneous mutation in the gene$$HER$2,$ a tumor suppressor gene associated with breast cancer. Mutations happen at a rate of about $1$ error per $$bases per generation. A spontaneous deleterious mutation must occur in$$HER$2$specifically$,$ and not in another gene. The likelihood that a spontaneous mutation is deleterious and could lead to cancer depends on the number of mutable bases in$$HER$2,$ the fraction of bases that will affect the function of$$HER$2,$ and that both$$HER$2$alleles are$$mutated$.$

A single cell will likely require mutations in both alleles of cancer$$associated genes such as$$HER$2$as well as a few other genes to cause the spontaneous cellular growth that leads to a tumor. Therefore, the likelihood of developing cancer from a spontaneous mutation also depends on the total number of cells in the body and the number of times each cell has replicated. Each cellular replication is another chance for a single spontaneous mutation to occur. Considering these factors, the likelihood of developing cancer from a spontaneous mutation should be extremely$$small$.$

Select the statements that best explain why cancer in general is still prevalent among humans, despite the extremely small probability of developing cancer from a spontaneous$$mutation$.$