What is the main criterion that scientists use to decide where to place the boundaries between the major divisions of the geologic time scale?

Out of the following given choices;

A. major changes in the structure of Earth's layers

B. major changes that occur in the solar system

C. major changes that are recorded in the fossil record

D. major changes in the arr angement of the continents

The answer is C. Changes in the environment in prehistoric times can be best exhibited by the evolutionary changes of living organisms. This is best shown by fossil records. Changes in geological times are characterized by a high rate of varying individuals of a species and with high extinction rates.