Although mutations occur at a regular and predictable rate, which of the following statements is a possible reason that the frequency of mutation often appears to be low?
(A) Some mutations produce alleles that are dominant and are always expressed.
(B) Some undesirable phenotypic traits may be prevented from reproducing.
(C) Some mutations cause neutral phenotypic changes that linger in the gene pool.
(D) The predictable rate of mutation results in ongoing variability in a gene pool.