Answer:
A) A molecular clock is a measure of evolutionary time based on the theory that specific DNA sequences mutate at constant rates. To use a molecular clock, scientists first select two different species and compare their DNA sequences. They can compare the DNA sequences directly, or by looking at the RNA and protein molecules created from the DNA. Molecular clocks help to measure the number of changes and mutations which accumulate in the gene sequence of different species over a period of time.
B) Molecular clocks assume the neutrality of gene mutation. They are based on the fact that genetic mutations although random, occur at a relatively constant rate. Evolutionary biologists use this information to deduce how species evolve and determine when two species diverged on the evolutionary time line. It is useful in the study of variations in selective forces, and also facilitates establishing the dates of phylogenetic events including those not documented by fossils. Molecular clocks do have some limitations. Rates of change may not be constant, may not be the same in all organisms, and may be different for different genes.
I used the answer above and added some stuff here and there, so credit to them. I'm not sure if what I wrote makes much sense, but hopefully it can be somewhat helpful. :)
