Question

which unit of geologic time is the oldest?
cenozoic
mesozoic
ordovician
precambrian

Explanation:

The Precambrian is the earliest and longest - lasting part of Earth's history, preceding the Cambrian period. The Cenozoic and Mesozoic are much later eras, and the Ordovician is a period within the Paleozoic era which comes after the Precambrian.

Answer:

Precambrian