What is often a problem when calibrating a molecular clock? A. There are frequently not enough negative mutations to accurately calibrate the clocks.
B. There are many different clocks, each of which "ticks" at a different rate.
C. The clocks use mutations that are difficult to identify and track.
D. The frequency of crossing over is used to calibrate the clocks.​

C. The clocks use mutations that are difficult to identify and track.

Explanation:

c.) polymerase chain reaction

- "unlike the original dna fingerprinting method, dna profiling does not use restriction enzymes to cut the dna. instead it uses the polymerase chain reaction (pcr)? to produce many copies of specific str sequences. pcr is an automated procedure that generates lots of copies of a specific sequence of dna."

meteorologists would compare a new weather map with one 24 h old to see how fast a front is moving.