Question

which best describes eras and periods?
○ eras have a smaller time span than periods.
○ eras and periods are independent of each other.
○ eras and periods both appear at the start of the cambrian period.
○ eras were more recently established on the time scale than periods.

Explanation:

1. Analyze Option 1: Eras are larger (longer time span) than periods, so this is incorrect.
2. Analyze Option 2: Eras and periods are part of the geological time scale hierarchy (eras contain periods), so they are not independent, incorrect.
3. Analyze Option 3: The Cambrian period is part of the Paleozoic era, so both eras (like Paleozoic) and periods (like Cambrian) are relevant around the start of the Cambrian (as the Paleozoic era includes the Cambrian period), this is a correct description.
4. Analyze Option 4: Periods and eras are established based on geological history, and there's no basis that eras were more recently established than periods, incorrect.

Answer:

C. Eras and periods both appear at the start of the Cambrian period.