Which of the following best explains a way that a molecular clock can be used?

a.) the number of dna mutations in an organism is measured over time to determine how long it will take for a new species to evolve.
b.) the number of dna mutations in a species is compared to the number of dna mutations in another species to determine the relative amount of time each species has been evolving.
c.) the number of differences in a specific dna sequence of two species is multiplied by a known mutation rate to determine the number of years of evolution that separate the two species.
d.) the number of differences in the dna sequences of two organisms of the same species is averaged to determine how many years of evolution have occurred

providing your proof would be nice : )