Geologic history is divided into eras, periods, and epochs based on the..?

The geologic time scale is divided into eons, eras, periods, and epochs (EHP-uhks). Unlike divisions of time such as days or minutes, the divisions of the geologic time scale have no fixed lengths. Instead, they are based on changes or events recorded in rocks and fossils.

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].

I'm pretty sure it's c. extremely gradual but not 100% sure though.