The molecular clock has limited usefulness for estimating divergence times among species in part due to the saturation of dna sequences. this occurs because

a. dna sequences are unable to revert back to previous allelic states.
b. the variation in the substitution rate between closely related lineages prevents accurate estimation of divergence time.
c. variation in population sizes among lineages changes the effect of genetic drift.
d. in highly diverged lineages, substitutions will occur at sites that have been substituted previously