The time units used by scientists to divide earth's history are part of the
a. 24-hour model.
b. 180° longitude.
c. geologic time scale.
d. 360° latitude.

The time units used by scientists to divide Earth's history are part of the geological time scale. The geological time scale measures the timing and the relationships of specific occurrences in the history of the Earth. It is used mostly by geologists and paleontologists. They define the earth's history by studying the Earth's composition then connect facts to major geological or paleontological events. 

Through the use of a geological time scale, scientists have found discoveries and put together the missing pieces of the past. Probably, the most distinguishable event discovered through the geological time scale is mass extinction of species.