What did you observe about the length of the eras? What geologic events typically mark a change in eras?

a) Eras vary greatly in length, and changes are typically marked by mass extinctions

b) Eras are relatively consistent in length, and changes are typically marked by volcanic eruptions

c) Eras are consistently short, and changes are typically marked by asteroid impacts

d) Eras are consistently long, and changes are typically marked by ice ages