The Intervals of the Geological Time Scale (Eras and Periods)

The intervals of the geological time scale (eras and periods) are not equal. This is because their length is based on

A) radiometric dating, and radioactive isotopes do not decay at a constant rate.
B) fossilisation processes, which have not occurred in the same manner, nor at the same rate, in all parts of the earth.
C) the positions of the continents, and the rate of movement of continental plates over the earth's surface has not been constant.
D) sediments, which accumulate at different rates dependent on the environment.
E) significant biological events, such as extinctions, which have not occurred at regular intervals.