Question

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

Explanation:

The Precambrian is the earliest part of Earth's history, preceding the Cambrian period. The Cenozoic is the most recent era, the Mesozoic came before the Cenozoic, and the Ordovician is a period within the Paleozoic era, all of which are younger than the Precambrian.

Answer:

Precambrian